Thats what I am trying to say. You disallow third party clients from joining lyrical servers because federation bad and fud about "brand".

Saying here you go. Open source. You can fork a client but dont connect, you can fork server but dont use official apps.

I do not know if GPL code can have this restriction. Its like saying Tesla car has open source or Foss software. Free to fork it but you need to build your own car to run this software. Its free, is open source, just not usable.