When I try to signup as username "Rusty" on https://idm.wikimedia.org/signup/, it gives an error saying "Invalid username, may already be in use"
How do I know if the username is taken, and is usurping possible?
Topic on Talk:Developer account
You can browse https://wikitech.wikimedia.org/wiki/Special:ListUsers, but be aware that not all Developer accounts have been attached to Wikitech so you may still not be able to use a name that is unlisted there. There is no mechanism for usurping the name of a Developer account even if that account has been dormant for years.