This javaagent repeats the logic of the combo-auth mod. it can run on:

• full-fledged servers (except fabric)
• bangeeCord
• velocity

using plugins or mods you can track where the player is registered from (authentication server) (example of a single possibility) ->

org.figrja.combo_auth_ahent.api.util.getPlayerBase().get(profileName).getLast()