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.
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.
The original server binaries were left on the original CDROM by a programmer.
Then PriitK, a creator of Kazaa and then Skype and Joost!, went on to re-create the client due to cheating/hacking, naming it Continuum.
Years later the server is reimplemented as A Small Subspace Server (ASSS), making it a complete fan remake of the original game (sans graphics). This is also when we finally got server side mods, everything before that was client only or a hack.
We even got on Stream Greenlight.
https://store.steampowered.com/app/352700/Subspace_Continuum...
We're lucky we got Priit to release the encryption/security module so Continuum clients could connect to ASSS servers without the security warning. I doubt it'll ever be updated, someone will have to take up the mantle.
No need to imagine. Pretty much all of that (minus the last part) happened in Amazon’s New World MMO in the first few weeks.
Though I wouldn’t be surprised if the last part did happen and we just didn’t know about it.
Either way, the point is that the difference between open-source vs close-source transfers is pretty significant.
No, it just means you need need to limit players to a trusted community - but that is usually how things work anyway because malicious players don't need any exploits to make a game unfun.
https://techcrunch.com/2023/02/28/gamers-are-fixing-a-video-...
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)