Microsoft has slowly been transitioning its franchises to PC over the last few years, thanks to its Play Anywhere initiative, which brings games to Xbox and Windows 10 PCs both, but the one glaring exception to the cadence of Xbox games on PC has been Halo proper. While Halo Wars 2, and Halo‘s Forge, have both been brought to PC, the mainline Halo games have not, so far.
This has caused some modders to attempt to replicate the experience for themselves on PC, with modder ElDewrito working to recreate classic Halo on the platform. That project was shut down by Microsoft- but in the statement that accompanied that, Microsoft said, which Phil Spencer reiterated later, that “As we look ahead, we’re very excited about the prospects of an official classic Halo experience making its way to PC and we hope to be able to partner with the ElDewrito team and broader mod and content creation community.”
Which seems to be a very explicit statement of intent of bringing “classic Halo” to PC- probably Halo: The Master Chief Collection. I assume this will be an announcement that comes this E3, then. For now, if you want to play old school Halo, Xbox One is your best bet.