The old, old, formatted Mystic Manual is under /mystic/docs/mystic.txt It is old, some is no longer relevant, but it's a start. After that look
up things at the wiki http://wiki.mysticbbs.com/doku.php between the two and whatsnew.txt (also in docs) that's all the documentation there is ;)
So at that point it's best to ask here since it is the mystic echomail area.

--- Mystic BBS v1.12 A47 2021/05/17 (Raspberry Pi/32)
* Origin: Mystic Pi BBS bcw142.zapto.org:23 (21:1/145)