Steam also has usernames that can't be changed. They added changing the actual email address associated with your account, but your original email address as account name is frozen forever, for old accounts.
Since 2003!
replies(2):
Since 2003!
Some tangently related anecdata: VRChat has a pretty strict email-changing policy. You need to confirm you own both the VR account and the original email account. Reasonable, but can be tricky if you don't have access to the original email anymore. I was able to navigate it but yeah, it's not always simple.