Halo: Reach is now playable on PC... for a handful of very early adopters. Microsoft and 343 Industries have shared a slew of Master Chief Collection update news, including word that it has started a public test of Reach on Windows PCs that lasts through July 1st. Don't get your hopes up for joining in this round, though. There are less than 1,000 Halo Insiders involved, and they're playing just one campaign mission. The developers plan to invite more people "over time," though, so you can consider this the start of a much larger test experience.
As it is, there's good news for players when the finished Master Chief Collection update arrives. While it Forge won't be available on launch for PC, the team is planning a "one-time" transfer of legacy Forge maps and game types from players' file shares to incorporate them in MCC. If you had a favorite Halo 3, Halo 4 or Halo: Reach custom map, you might get to play it in a modernized form. There's no specific date yet, and there doesn't appear to be a "workable solution" to bring films and screenshots to MCC, but it beats losing all your old content to the mists of time.
The devs also clarified plans for an MCC-wide progression system. Unlike some season-driven games, you won't lose the chance to unlock gear from one season once the new season starts. You can even use points earned in one game toward customization in another. Also, Reach may include ways to unlock content that required promos the first time, such as pre-orders and account links. Although there's no guarantee you'll see everything you ever had, you probably won't be limited to the base version of each game.