Ever since the finally got added earlier this year. Now the team is expanding that with a bunch of new mod tools coming in Halo MCC's Season 7 next week.
The most recent mod tools for the Master Chief Collection on PC were added in April as part of Season 6, a recreation of Mario Kart within the game.
At the time, the team stated that its goal for Halo MCC mod this year "is to roll out and improve the modding experience on PC" before 343 looks into possibly bringing mods to the console version. The next step is about to happen this coming Wednesday, June 23rd, as a new set of expanded mod tools will be added to the Master Chief Collection on Steam.
At the moment there are no further details on what these new mod tools can do, although 343 promises to post an update on its website "that dives into what they are and how you can use them," which will presumably appear before the new tools themselves drop on Wednesday.
https://twitter.com/Halo/status/1405149551409852416
The Halo: Master Chief Collection has certainly been an enormous success since finally coming to PC, as the multiplayer will be free.