Shiren the Wanderer GB was developed by a relatively unknown Japanese company named Aquamarine, although all subsequent re-releases were developed and published by Spike Chunsoft.
The Swordswoman Asuka Arrives! was developed by Neverland Company Inc., creator of the Lufia and Rune Factory series. The company's demise in 2013 seems to be the reason why Asuka Arrives has never seen a re-release and has been shuttered into Canon Discontinuity territory.
Dummied Out: The Sending and Receiving Jars In the Japanese versions of Shiren 3 were used to respectively send items to and receive items from other players, but Atlus removed all Wi-Fi functionality. If hacked into the English Wii version, the items' descriptions even say, "Hacks! This jar is not supposed to appear!"
The original Super Famicom version of the first game received a fan translation courtesy of Aeon Genesis.
A fan translation of Shiren 2, which was two and a half years in the making, was also released to commemorate that game's 21st anniversary.
Keep Circulating the Tapes: Save Surala was only ever available on the Satellaview for a few weeks in 1996, and has never had a permanent rerelease. Collectors managed to partially preserve it (specifically, Weeks 2 and 4), but weeks 1 and 3 are unfortunately lost to time.
No Export for You: All the other entries besides Shiren 1, 3, 5, and 6 have not been officially released outside Japan. But it's surprisingly easier to import this particular series than most: even if you do understand Japanese, it's mostly just trial and error. Plus, most of the other games are on region-free consoles like the DS.
Sequel First: Shiren 2 and 4 were not localized, so the West has only ever gotten four of the six numbered titles. This technically also applies to the first game since it was prefixed as Mystery Dungeon 2 and the first Mystery Dungeon never left Japan, either.
Sequel Gap: Shiren 6 for the Nintendo Switch came out 14 years after the original DS release of Shiren 5.
Word of God: According to this tweet from Chunsoft's old official Twitter account for the Shiren series, there is a secret behind Koppa's backstory in Shiren 2 (having befriended 10-year-old Shiren some time before that game) being consistent with his original backstory in the first game (having traveled with 18-year-old Shiren for six months after meeting him for apparently the first time).note According to the Japanese fandom, the secret is hinted at in GB2...
Other trivia
Historically, Xu Fu vanished in 210 BCE during the second expedition to find the elixir of life for Qin Shi Huang. The main plot of Shiren 3 occurs exactly 2,000 years later, which would date it to 1790 CE. Stemming from this concrete date, a few other estimates can be made:
272011 BCE - The Tower of Fortune's previous challenger.
788 CE - Kokatsu's victory over Suiryu.
790 CE - the events between Shiren's ancestor and Kaguya that would become the basis for The Tale of the Bamboo Cutter.
from Chunsoft's old official Twitter account for the Shiren series, there is a secret behind Koppa's backstory in Shiren 2 (having befriended 10-year-old Shiren some time before that game) being consistent with his original backstory in the first game (having traveled with 18-year-old Shiren for six months after meeting him for apparently the first time).note 