I'll post this video popular Valve-centric YouTuber Tyler McVicker uploaded which details the situation a bit more:
But basically there's been a recent situation where Valve has requested development cease on two fan projects involving Valve IPs, a very odd move for Valve as they've famously been known for being cool with many fan projects, including ones people can even sell for profit, for better (Black Mesa) or worse (Hunt Down the Freeman).
First Valve has asked the dev behind the "Portal 64" project to cease development on said project, and in this situation it sounds like Valve asked for development to cease in order to protect themselves from the possibility of Nintendo taking issue with the project possibly using resources that can be traced back to Nintendo (this sorta goes back to something I said when Valve stopped Dolphin from being put on Steam, people keep calling Valve "the most pro-consumer company out there" but stuff like the Dolphin situation and now Portal 64 show they're absolutely NOT willing to back fans if it means they, Valve, also get dragged into a court case, as at the end of the day Valve is still a corporation that doesn't want to spend money on what they feel isn't worth it).
The next one is Valve sent a DMCA to the github page of a project that was aiming to bring Team Fortress 2 into the newest game from Gmod creator Gary Newman, S&Box. Not much is known on why Valve sent a DMCA, but due to a combination of this and coding issues the team that was working on this fan project have opted to cease development.
If I may be a petty bitch for a moment I can't help but feel some mild schadenfreude towards people who are 100% unironic in being part of the "PC Master Race." They love to boast how much Valve are "cool with everything" and say stuff like "bro Nintendo ain't your friend, stop simping for them" anytime someone expresses admiration towards the games made by a company like Nintendo or anytime it's Nintendo issuing a DMCA, but bruh stuff like this shows Valve is 100% no better than a company like Nintendo and if "Shigeru Miyamoto doesn't know you exist or care about you" then same to you about Gabe Newell, he ain't your cool uncle.