Doxxing is a low skill level threat its very easy, google your target’s username and find information about them, yeah its that simple. All your user data is linked to your user name and profile picture. What i suggest doing is using the same tools doxxers use (sherlock) to find your long forgotten accounts and then delete them, remember to never use the same username and profile picture.

When a database leaks it will most likely contain your username, email and ip address, this information will be findable by username, email or ip address. If you ever use an account without a vpn using a vpn wont anonymize you from the service. Disable webrtc in your browser it often leaks ip addresses while using vpns also watch out for ip grabbers.

Tldr: dont use same usernames and profile pictures, vpn is 100% secure if used wrong.