Apparently mod support is coming to Halo: The Master Chief Collection. If you installed this game (either by purchasing it on Xbox or Steam, or by installing it with the Xbox Game Pass) then you currently have access to Halo: Reach. The other remasters are still in development.
Attempting to launch it shows two options: the game, and the game with anti-cheat disabled for mods. This was the first I heard that Halo: Reach would get mod support, although Polygon is reporting that 343 Industries is working on tools that will arrive “shortly after the game”. Disabling anti-cheat will disable rank matchmaking, which (reasonably) limits mods to custom games or the campaign.
I am looking forward to the mod tools, but cautiously. Halo 2: Vista, although from a different time and a much different Microsoft, technically had mod tools. Those tools were so locked down that their own example map was impossible to create (due to assets that were removed before launch and cannot be replicated through the tools).
On the other hand, Halo: Custom Edition was one of the best PC gaming experiences that I’ve ever had (despite its horrible netcode). The handful of popular, third-party maps were great, and the Halo slow-ish paced mechanics (due to shields) was a great offset to the chaos of Unreal Tournament. Both games complemented each other quite well for that window between ~2003 and ~2007… at least for my taste in PC shooters.
Halo: Reach launched today on Xbox/Microsoft Store and Steam.