Q: Is it correct that no database migration has to be executed when upgrading from 1.9.4 => 188.8.131.52?
A: Yes that is correct.
Q: do I need to take additional steps to make sure the Oauth2 plugin keep functioning?
A: As far as I know the only thing necessary would be to download the update for the oauth plugin as well. When there is no update for the oauth plugin, there is no need to do anything.
That said a little additional note: Since we personally never used the oauth plugin I don't how it works but since we use http auth and ldap auth I guess it's the same procedure - update osTicket, update the plugin (when there is a update too)