The mechanisms by which I check if a username is taken on this page are pretty rough and ready.
I don't use APIs to check availability - instead I check the page response for a site - so e.g. if I visit example.com/username and it returns an error to say the page doesn't exist, then I assume the username is available.
This method isn't at all foolproof, so just because the results say a username is taken, doesn't mean it really is.
This is a bit of an essay, but I just wanted to say that this is a rough and ready checking system.
You can double check the results from this page against other username checking sites, such as: