API Username Check Bug?

As many probably know, the old API endpoint was removed which checked to see if a username was valid. I have been trying to change my username to "polar", but it seems that it does not know how to check for a username anymore. My guess is one of three reasons:

1. The old endpoint was removed and never replaced. Not likely, as why would they remove a key piece in joining the site.

2. The new endpoint just takes a long time to load. Would not be surprised, from what I have heard of this new update, everything is terribly slow. 

3. You can't change your username more than once. I have changed it before, and so perhaps I am not allowed to change it again? If so, it would be nice to get some kind of message: "You can't change your username more than once."

Thank you.