Valve has issued two DMCA takedowns and brought the hammer down on two popular mod projects in what's widely regarded as a surprise move for the developer, publisher, and Steam platform owner.

The incredible Portal 64, which got Valve's iconic game running on Nintendo hardware, is no more.
Portal 64, a project that ports Valve's iconic first-person puzzle game to an unlike source - the Nintendo 64 - is one, and Team Fortress: Source 2, which updates the classic shooter with the newer Source 2 engine, is the other.
We previously reported on Portal 64, the ambitious and impressive project from programmer James Lambert. Considering the age of N64 hardware, the fact that it works at all is incredible - with the most recent update to the project confirming that a large portion of the game was complete and up and running.
The GitHub page for Portal 64 is no longer up - days after the developer completed an impressive milestone. After years of tinkering and programming magic to get the game running, James Lambert confirmed that Valve contacted him to take the project down. The reason? Portal 64 depends on Nintendo's proprietary libraries. So this could be a case of Valve being cautious, as Nintendo is notoriously unfriendly (see: litigious) regarding mods, emulation, and ports.
Team Fortress: Source 2 is a different story because it's Valve's IP - but even so, the company issued a copyright takedown request because the mod uses Valve's assets. It's a strange turn because Valve has supported mods, ports, and remakes in the past, especially for its games - going so far as to support them monetarily on Steam.
Is this a new era for Valve and modding? It's hard to tell, but the company targeting these two high-profile mods and ports is not going down well with the community.