The developers at Lionhead Studios have already confirmed that their upcoming addition to the Fable series won't be come "ghetto PC port," but will be a full-fledged game built specifically for the PC. On top of that, the PC version and the Xbox One version will feature cross-play.
Other than that, there hasn't been much on the game's launch or any other details around that. It seems as though Fable Legends will not be available to all PC users — only to those with Windows 10.
This new information was gleaned from a Twitter conversation between Lionhead's Community Managers and a Twitter user.
@sertificationNu I think you may have answered your own question there 😉 It will be on Windows 10 but not Steam, sorry!
— Lauran (@LHAlbionGirl) August 24, 2015
So, there's that unfortunate bit of news.