In a couple of weeks, the site is going to undergo a huge overhaul. Sigil Prep is about to become a whole, independent, OGL-compatible setting. It’ll still use 5e rules and it’ll draw from the SRD, but the intellectual property within will be purely mine.
No more teachers from the Forgotten Realms and Ravenloft, no more references to Eberron and Mystara. But! It’s going to be more creative, more descriptive, and more professional looking.
All the critical stuff will still be on this site for free, including new races and subraces and dozens of new subclasses, with more detailed stuff will follow, including backgrounds, equipment, spells, monsters, a new cosmology and the gods that go with it. Once the core setting is completed, specialized booklets detailing certain elements of new Sigil Prep will hopefully follow on DriveThruRPG.
In the meantime, if you like the existing Sigil Prep, I would certainly encourage saving the documents. They won’t be available much longer.