In a multiplayer game, the client renders and accepts inputs.

Then, you can also just freely release the server hosting software, as was the norm in gaming, for decades.

Ever hosted a private server for anything? Batllefield 2? HL2DM? Quake 3? Minecraft? Conan Exiles? Arma 1 2 or 3? etc?

Shit, even the original Deus Ex has a mutiplayer component, that you can host a private server for.

There's no reason that a client being subservient to the server, in a multiplayer setting, means that nececassrily you don't control the game client.

What you are thinking of is a 'thin client', where the client is now not even doing rendering of the game, its just accepting player inputs, and your gameplay is actually you watching a livestream being rendered somewhere else.

Thats not the same thing as deisgning the client and server in a clever way such that the client cannot force things onto the server that it should not be able to.

Yes and no. This very much depends on how the game is implemented. In an open source game, the code is still yours to edit to your heart's content, it's just that if you're a client you'd be bound to what the server says happens.

In a closed source game, if the server executable is released, there is at least a degree of ownership there.