I just created an account for the forums.
While normally i use created passwords with more than 16 characters, I was not able to do so for the forums.
I don't now the exact max length, as i haven't tried them all.. :D
The problem I had (besides the restriction itself) was, that there was no real information of the signup form of the error.
I had to manually look up the request via dev-tools in order to get to know the error.
There should be at least an error indcating, that the password is too long.
Or a text, that shows this limitation statically.