Apple supplied some, were there others that other people wrote? What is the complete list of FSTs available?
The official ones, with GS/OS versions as indicated by the Apple II History site:
Filename Purpose In GS/OS Version APPLESHARE.FST For AppleTalk networks 5.0 CHAR.FST Keyboard, screen, printers, and modems 4.0 DOS3.3.FST Apple II DOS 3.3 Disks (Read only) 6.0 HFS.FST Mac OS HFS Disks (Read/Write) 6.0 HS.FST High Sierra CD-ROMs 4.0 MSDOS.FST MS-DOS Disks (Read only) 6.0.1 PASCAL.FST Apple II Pascal Disks (Read only) 6.0 PRO.FST Apple II ProDOS Disks (Read/Write) 4.0
Apple didn't release documentation on writing FSTs, perhaps because they often involved changes to GS/OS internals. This is discussed at the Apple II History link above.
However, the following unofficial ones were developed by Brutal Deluxe:
DOS3.2.FST Apple II DOS 3.2 Disks DOS3.3.FST Apple II DOS 3.3 Disks (Improved) RDOS3.3.FST Apple II RDOS Disks (SSI games)