Trailing underscore in username is now allowed:
https://github.com/discourse/discourse/commit/b3d2982be7d40ff632afcd1c3cbe8c9e78a632be
However UserNameSuggester still suggests usernames with a sequence of 2 or more special chars (like arpit__jalan) which is not valid. I am working on a fix for that. Fixed:
https://github.com/discourse/discourse/commit/d73d4d476984d6de647423d3869cfdb1d72c427a