Of note, GitHub doesn't allow periods in usernames either. I'm not a Ruby expert but I wonder if file extensions give some specific Ruby gotchas that means both GitLab and GitHub operate this way.
It's not a Ruby gotcha, it's a Rails routing gotcha. You can specify alternative formats on any path, e.g. you can access /path to get HTML (or whatever the "default" format for that controller method is) or you can access /path.json or /path.xml and if the controller method specifies handlers for those formats, you get that format. So if you allow a username like "john.doe" and the route is something like /john.doe then Rails will interpret the "john" as the ID part of the path and "doe" as the format part of the path. You can override this in your routes to support periods but then you do lose the capability of accessing alternative formats which sometimes can be useful.
Thanks. Now I also get a better understanding of why .patch for MRs or .keys for user names as file extension work on both Rails platforms. I always found these file extension hacks very useful for quick access and automation.
I could imagine the mix of rails #respond_to and "file extensions" at the end of urls might make a mess (think /users/profile/smith.html vs /smith vs /smith.json vs /smith.txt - essentially what might have been /smith?format=json etc).
