no one knows why

Well the reason for not allowing a regex symbol in the user name seems pretty clear to me.

EDIT: and if you mean the appendix then it's a vestigial organ that is used to digest non-starch polysaccharides in ruminants but we use it as a storage area for commensal micro-organisms.

Is it normal for Devuan to be unable to enter a username containing a dot?

This is what i get when i try to add a username with a dot.

~ > sudo adduser john.doe
[sudo] password for user1:
adduser: Please enter a username matching the regular expression configured
via the NAME_REGEX configuration variable.  Use the `--force-badname'
option to relax this check or reconfigure NAME_REGEX.

So default from installer would most likely not allow it, but you could perhaps do it manually after install.

Just reading up on this and it is due to POSIX compliance.

Have a read here: https://paulgorman.org/technical/presen … ntions.pdf

quite an interesting read.