Atlee (aka Terra III) after the writer/artist switch during Power Girl's solo series. She appeared in one panel of the first issue after the switch and was then never seen again. Especially noticeable as she had basically been Kara's sidekick and was well-liked by fans. She later made her return in Starfire (2015), done by the same writing team that introduced her in the first place.
In the 70s Batman comics, Lucius Fox had a delinquent son named Tim. The character disappeared very quickly, even though his sister Tiffany continued to show up. Post-Crisis, Tim only ever made one appearance (a cameo in an issue of Gotham Knights), and continued to be forgotten about after subsequent writers introduced two new Fox children: Tam and Luke (the latter of whom became the second Batwing). The character's continued, coupled with nobody ever even mentioning the poor kid, caused some fans to speculate that Flashpoint had wiped Tim from continuity altogether, with Luke retroactively replacing him as Lucius' only son. Tim finally reappeared 18 years after his last canonical appearance, with an altered backstory, having changed his first name to Jace: he first appeared as the Batman of the future in DC Future State, and then was shown originally gaining the title in The Next Batman: Second Son and I Am Batman.
Superman's Post-Crisis supporting cast were built up over such a long stretch of time, and then summarily jettisoned in 1999 when the Dan Jurgens/Louise Simonson era ended. Perry White's family, Bibbo and the other characters from Suicide Slum and the Bottle City characters introduced during the '90s all vanished abruptly. Other characters, like Emil Hamilton and Maggie Sawyer, got thrust deep into the background overnight (Maggie soon moved to Gotham).
Supergirl's supporting cast and Rogues Gallery is often jettisoned and replaced every time a new creative team takes over. Dick Malverne, her foster parents Fred and Edna, some of her enemies such as Lesla-Lar, 'Nasty' Luthor and Black Flame were all never seen again after the Crisis on Infinite Earths in 1987.
Supergirl (Rebirth) begins with Kara getting adopted by Jeremiah and Eliza Danvers. In issues #34-35, which were 'DC Year of the Villain/Event Leviathan'' tie-ins, Kara finds out that Jeremiah is fighting against a secret organization called Leviathan and that his wife Eliza joined them against him. Supergirl is then foced to leave Jeremiah in mid-sentence to answer a distress call which kicks off the "Infected" plot, and Jeremiah, Eliza and Leviatan are forgotten. The next time we see Supergirl -after her series' ending- she is living on her own in Metropolis, and she never ever mentions her adoptive parents.
In the 90s, Lex Luthor had an infant daughter named Lena Luthor, who he loved very deeply but she nonetheless got roped into his life as a supervillain against his wishes. After 2004 though, his character was sent into a different direction. In the midst of it all, Lena just disappeared and it was never explained what happened to her or where she is now, leaving Superboy as Lex Luthor's only biological child. Her disappearance was eventually explained. As a result of the Cosmic Retcons that took place during Infinite Crisis, Lena was Retconned back into being Lex's younger sister (which she had been Pre-COIE) rather than his child. Lena's daughter Lori subsequently became a prominent supporting character (and love interest) in Superboy, at least until she was removed from existence in the nextCosmic Retcon.
In Superboy (1994), Kon had a dog named Krypto; it was a rebooted version of the Pre-Crisis Superboy's childhood pet, a superpowered Kryptonian dog sent by Jor-El before baby Kal-El was sent off in his rocket. Krypto the regular Earth dog was never seen again after Superboy brought him back from Hawaii to Cadmus headquarters. Not long after, Superman actually retrieved his childhood pet Krypto the Superdog from Krypton in 2001 "Return to Krypton", and early on in Geoff Johns' Teen Titans this dog was given to Superboy as a pet – somehow it never came up that he used to have another dog with the same name.
The Flash: One of the most popular features of Bill Messner-Loebs' run was the large supporting cast — they even carried the book without Wally for a couple of issues. When Mark Waid took over, all of these characters except Linda Park and Piper faded into the distant background as Waid proceeded to assemble a new "Flash family". Waid's run was wildly successful, but longtime readers still regret the loss of those characters. Chunk later returned as part of the "Anti-Eclipso" task force in that villain's book but avoided being subsequently murdered along with several other mostly forgotten characters, like the female Wildcat. He'd later show up at the beginning of Geoff Johns' Flash run targeted by Amnuet Black.
Robin (1993): Dana Drake, the stepmother of Tim Drake. When Tim's father Jack was killed off in Identity Crisis, it was done so Tim could be tied more closely to Bruce, who would formally adopt him. The problem was, though, that Jack had been remarried to Dana, who had been a major supporting character all through Robin's own series. Dana had a mental breakdown over Jack's death and was sent to a sanitarium to recover... In Bludhaven, which would soon be completely destroyed in Infinite Crisis. One could assume Dana died along with everybody else in Bludhaven, but it was never explicitly stated that she did. Instead, she has simply never been mentioned again. Evidently, DC editors wanted to get rid of her, but realized that having Tim Angst over losing both his father and stepmother in 2 close together yet completely isolated incidents (especially when his girlfriend and best friend has also just died) was too much. She would be a Forgotten Fallen Friend, except that it was never confirmed she had fallen.
The Impulse supporting cast soon vanished after his book ended, most notably Bart Allen's best friend Preston and love interest Carol. Much to fans' displeasure, one line in Geoff Johns' Teen Titans had Bart suggest it was Wonder Girl who got him to like girls, rather than mentioning Carol or even Arrowette (another crush of his). Bart's pet dog Dox was also never seen again or mentioned by any later writers.
Holly Robinson was one of Catwoman's prominent supporting cast members, even temporarily taking on the codename for an arc in the One Year Later era. However, after the "Countdown" event, Holly vanished and was never seen again in the Batman universe. A later throwaway line about the character suggests that she moved elsewhere after receiving a huge sum of stolen money from Hush's fortune. This is in no small part because of the sorry reputation her creator, Frank Miller, picked up over the years–the makers of The Dark Knight Rises even went so far as to create a similar character called Jen to avoid having to forward any money to Miller.
Between 2001's Officer Down and Face the Face, the One Year Later Time Skip tie-in to Infinite Crisis, Jim Gordon had retired from the post of Gotham's police commissioner with Michael Akins taking over the post, who didn't want to rely on Batman so much and eventually cut off the GCPD's ties to Batman after the events of War Games. Come Face the Face, Gordon is back as Commissioner (and subsequently the GCPD resumes its alliance with Batman) with no real clue about Akins's fate. However, The Bus Came Back as Detective Comics (Rebirth) saw him as Gotham's new mayor.
Post-Crisis Wonder Woman entered a business partnership with her friend, Boston-based PI Micah Rains. Without a single word as to what happened to their partnership and the offices she rented for them, she then moved to the other side of the country. He was never mentioned again, despite her returning to Boston on several occasions.
This is pretty much true for most Wonder Woman characters which were not created by George Pérez or the original Golden Age creators. There are tons of love interests, supporting cast members and villains who simply never appeared again after their respective creator left the series. The few exceptions would be characters like Phillipus and Artemis, and even they seem to have been dropped as of the New 52revamp.
After Phil Jimenez left Wonder Woman Cassie's best friend George was never seen nor mentioned again, despite her lasting through a couple of writers on Wonder Woman. Even at points when Cassie's friendships were part of larger stories elsewhere it seemed the writers and editors all forgot she had any outside the costumed superhero community, or didn't want to bring up Cassie's tomboyish origin and best pal after she was revamped into a very feminine girly-girl by later writers.
Blackhawk: The early stories in the 1940s featured three squadron members named Zeg (Polish), Boris (Russian) and Baker (English). They all vanished without trace after their initial appearances.
Boris DID return briefly in the short-lived mid-1970s revival.
Tomahawk: After the Retool that introduced the Rangers, Tomahawk's sidekick Dan Hunter popped up less and less frequently until he disappeared entirely without explanation.
Hunter's Hellcats: Once the feature settled on the main criminal characters of Brute, Swinger, Snake Oil and Juggler, the other Hellcats just kind of disappeared, apart from the occasional Red Shirt. Sometimes unnamed Hellcats would appear, usually brawling with the main characters, so one possibility is that these are the name characters from the early stories.
The New 52 introduced a new version of The Question who was a faceless immortal who had his memory of his past erased due to committing some crime. After appearing in the "Trinity of Sin" miniseries, the character vanished and was never seen again. A few years later, the original Vic Sage Question and the Renee Montoya Question were brought back while the New 52 Question is still MIA.
