My knee jerk response to this is definitely influenced by how companies like Nintendo issue takedowns, which was to get mad in a "they can't get away with this!" sort of way. But actually reading further...
Development on Team Fortress Source 2 had stalled out because it was being made in "s&box", basically the next-generation replacement for Gmod running on Source 2. s&box is still early enough that parts of it are changing wildly from version to version, which made maintaining Team Fortress Source 2 something of a nightmare. They were trying to open the code up so others could continue working on it, given the original devs were also starting to run low on having the free time to maintain it. I'd actually been following this one for a while. Development on the project had already been stopped for the better part of a year. There isn't a reason stated why Valve is DMCAing it now, but the project has been in limbo for a while so it stings less, I think.
The DMCA takedown for Portal 64 makes a lot more sense given it sounds like Valve wasn't comfortable with the fact the project apparently uses versions of Nintendo's proprietary code libraries. That's something that could cause actual legal problems, because that means they were probably pirated from the gigaleak. It'd be a real headache if Valve ever wanted to get behind Portal 64 in an official capacity -- and chances are they did (it's what they do), and upon learning how it was made, made this call. It sucks, given how promising Portal 64 looked, and how much time that dude poured into the project, but I also understand why Valve is doing this.
it's unfortunate that he was using the leaked SDK for this. judging from the in-depth explanations in his videos, i think he has the skills and know-how to potentially make something like Portal 64 atop the fully open-source libdragon SDK instead.
