Long lasting usernames across websites is the worst for privacy though, unless the username is not public. In general, it's best if the unique identifier is only known to the user.
I've seen quite personal details being leaked because sometimes even smart people don't realise how easy it is to cross-reference given a unique username.
Example: https://instantusername.com
I've seen quite personal details being leaked because sometimes even smart people don't realise how easy it is to cross-reference given a unique username.