BuddyPress.org

Lots of previous threads for this. Even a recent one with a Trac ticket to make it so it doesn’t require lowercase:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/register-username-with-uppercase/
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/allowing-users-to-type-in-uppercase-letters-for-the-username/
https://buddypress.org/community/groups/installing-buddypress/forum/topic/username-starts-from-uppercase-letter/

I’m not sure if the lowercase does matter in other sections. You might add the question to the Trac ticket and see if the core developers can answer. This is something I’d like to fix too. The other solution I’ve considered is to have some javascript function that will check the username to make sure 1. it’s available and 2. that it’s lowercase and 3. Any other validation. At least a notification before they click submit would be sufficient.