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.
This isn’t a technology problem. We just need to put our foot down as a society and make tracking as illegal as sexual harassment/assault and aggressively and visibly punish and shun companies that abuse you. There isn’t a technology that magically makes you safe for creepy internet stalkers.
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.