I recall some similar past discussions, not only about ! but other non-digit-or-letter characters which are/were permitted in creating a user name but which cause trouble in other BGG contexts.
Yes, the vertical bar thingy -- "|" -- is one of those. I *think* such special characters aren't allowed when making new member names now, based on what someone of the adminnish persuasion told me in a chat.
So these days the special characters gather in Chit Chat....