It doesn't need to be open source, you only need to provide server binaries to download. This was the standard until circa 2010. People were able to host dedicated servers themselves.

That would be an improvement over nothing, but closed-source means that the game is still going to die as soon as someone finds a security vulnerability (or even just a gameplay glitch) that can't be feasibly patched.

Imagine an MMO where special text in the chat causes viewers' clients to crash, or a glitch exists to duplicate items or money, or where anybody can crash the server to run arbitrary commands.

I want to host a closed search server that's not being updated on today's internet. It might be good enough for home use, but definitely not if I want my friends to connect.

Although I get the idea of providing server binaries but if one has to absolutely do it, then provide great modding efforts behind it.

But I have found that the greatest modding efforts/community can be generated by open source. Balatro for example is easily modified in the sense that although it might not be open source but iirc its lua files are visible.

There are other games as well which have something similar imo although that being said its possible to create modding efforts without open source in general too with say something like for example old versions of counter strike.

Personally I would prefer open source though if its possible but I understand that some game studious might be worried about it but I don't quite understand it if they are shutting down the game anyway though. I think that @mjr00's comments are nice about third party library etc. which cause issues in open sourcing so its good to have a discussion about that too (imo)

Closed source binaries rot.