what I find annoying is that (other than HN) most places won't let you claim your username if the person has signed up, yet never posted over years (or even logged in) and places where an account was deleted.

when I signed up to HN, I had a different username. I reached out to admins and they looked at the other account that had been created but they let me have it because the person hadn't logged in since. Oh! I think GitHub did as well. (shit, I wonder if I have mixed up GitHub and HN... I'm pretty sure they're the two that did actually let me have my handle... :x)