What you're noticing - that companies suddenly started closed-sourcing their server code, isn't just something that happened for no reason. Internet connections got fast, and it enabled more complex games like MMORPGs and MOBAs which are an order of magnitude more complex than the shooter games you're probably thinking of (Quake III?)

A LOT of architectural thinking goes into scaling MMORPGs, and it starts at the data model. I've watched quite a few interviews with WoW devs, and the data model for their system is counter-intuitive. That constitutes a trade secret in my mind.

And that's something that companies that produce MMORPGs/MOBAs/etc. can conclusively demonstrate in court.

That said, it's possible that companies could release a "nerfed" version of their server code with trade secrets removed for the community to run once the official game servers are taken down. It isn't clear if that'll be in scope of the law, or if that's even what proponents of Stop Killing Games would be okay with. We'll have to see.