Its a long story, the source code to falcon 1.07 was leaked and worked on semi-legally for many years. Microprose tried to shut them down, never successfully.
Another group of modders were working on the outside on the 1.08 game.
eventually both group of modders worked out a deal with the legal owner of the software which lead to the creation of Lead Pursuit. LP cleaned up the code and released F4AF in 2005.
F4 when it was a released in 1999 was a buggy mess with near-constant CTDs. By comparaison, SH5 looks like a 100% complete game...
I also have the F4 binder pictured above in my bookcase. It came with the special edition.