This app was mentioned in 3 comments, with an average of 4.00 upvotes
There are progressions, and pistol squats are advanced. There is a place for everything, and with bad knees, strong legs protect them. May seem like a catch-22, but it is all about doing what you can safely do and work up.
I'm doing the Start Bodyweight app and had to begin at a lvl 44, and in 8 weeks up to lvl 210 and still not to pistol squats yet (but hopefully Assisted Pistol Squats within the next week or 2.
https://play.google.com/store/apps/details?id=uk.co.byian.startbodyweight
there's an app for that. it's basically everything on the start bodyweight website in a tightly wrap package