Unused space spreadsheet I've compiled

Nice work!

Technically I think all entries are technically free space but a somewhat good amount are right after end of some data like NPC data, event triggers or chest data as an example. If someone put something there blindly assuming it would never be touched, an utility like FF6LE could erase the data by adding chests as an example. So as a safety measure, always check in the ROM map what precede the free space

This is a good point.  I used the FF6offsets.txt for the list of unused space to compile this so it does indeed include empty space at the end of some data blocks.