I've been looking through the files. It's not just AmigaOS 3.1, there is some older/historical stuff which is interesting to me. For example:
Source for the Amiga (1000) keyboard microcontroller: other/keyboard/keyboard.tar.z
ABasiC source code (ABasiC came with the first Amigas before AmigaBASIC): other/abasic/abasic.tar.z
The Tripos kernel of AmigaDOS (written in assembly language),
BCPL source for CLI commands and (maybe) the rest of AmigaDOS, changelogs: other/old_fs/old_fs.tar.z
What is probably some source for Metacomco Lisp (did Commodore market that?): other/lisp/lisp.tar.z
CD32 MPEG device source: other/cd32mpeg/
CD32 early ROMs (v40.5, 40.6)...
Apart from history, some parts could be useful, for example someone might want to debug/bug-fix scsi.device etc. Getting individual modules like that into a buildable state would be much less work than the entire OS source tree.