I have no experience in the field, but is it really that difficult to have servers on Xbox as opposed to everything else? I mean hell, some games fully support multi-player servers for all platform cross play.
The problem with cross console on private servers tends to come from the rules set forth by the company. Microsoft has a lot of restrictions on how cross platform multi-player can be implemented, and since the servers are being hosted by the players and not the company they most likely aren’t able to meet the requirements.
Can you link these restrictions?
Unfortunately Microsoft puts them behind a developer account and NDA requirement. The less specific docs can be viewed here but they tend to only make vague references to the restrictions (mentioning things like API service needing to be discussed and set up with your point of contact, etc.): https://learn.microsoft.com/en-us/gaming/gdk/_content/gc/live/features/multiplayer/live-multiplayer-intro
Is that a Pokémon holding an uzi with a banana clip?
That was a fun question to write, even if it is never answered ᕕ( ᐛ)ᕗ