The developers of a game similar to Among Us would be forced to update the game with bots to be compliant?

I'm not a programmer or IT specialist or anything but I think the ask would be more like,

when discontinuing multiplayer service they must roll out an update to allow gamers to specify private server addresses

They don't need to program bots, they need to open the code enough for fans to keep the game functional. Like when I was a kid I could play multi player games with my friends by typing their phone number into the game. Our computers would connect through the phone lines and we could battle for the fate of Azeroth. I still remember the phone number of the friend I did that with.

Open sourcing it is the solution but there is no enforcement possible

Studios can just shut down unless they are required to hand the source over to enter the market

I feel the same way regarding whether this legislation would be enforceable or good, but there are a lot of ways developers could make this work that they currently don't. That includes bot players, local multiplayer functionality, dedicated server tools, IP-based connections, etc. Many DRM and anti-cheat implementations also cause problems and would need to be either removed or only used in certain contexts.

Right now in a lot of games if you aren't playing multiplayer on official servers through official matchmaking functions with invasive kernel-level anti-cheat there's no other way to play, but that hasn't always been the case nor does it need to be the case.

You pose a good question. There are many ways that it can be implemented and of course it depends on the game. Have a solo/offline mode, replace online multiplayer with local multiplayer or direct connect (for games like Among Us), release a dedicated server (so groups of people can start their own.

At a minimum, it needs to be better than "Could not reach game server, please check your connection and try again." The standard we could hold companies to would be through each province's consumer rights agency, but would have the legislation properly behind their efforts.

Publish the source. Look at id and Quake/Quakeworld. The game is still alive and well after Romero released all source codes to let people iterate their own clients and servers.

I imagine it being basically impossible for MMORPG games. How do you keep an MMORPG alive without removing the MMO part?