Technically, Mac OS did not drop support for 400KB disks with Mac OS 7.6.x. With System 7.5.x, a lot of floppy disk handling shifted from the core OS to PC Exchange, or whatever it was called that day. So when you insert your 400KB disk into a Mac running OS 8.1, it is being rejected by PC Exchange, not core OS.

I found some hand written notes about 400KB disk handling a few days ago, so I'll post them over the weekend.