If you plug an external macOS system disk using APFS into a newer version of macOS you will get a warning afterwards back in your original macOS saying:
"Incompatible Disk. This disk uses features that are not supported on this version of MacOS"
How to reproduce: Install mojave to an external drive. When everything runs fine shutdown. Take the disk and mount it under a system running big sur. Eject the disk. Boot mojave again from the external drive and see the above error.
You don't need to do anything with that disk in the newer OS, mounting and unmounting again is sufficient. It does only happen to disks containing a macOS system.
I had that issue with several macOS combinations:
mojave disk -> mounting under big sur -> boot back to mojave
high sierra disk -> mounting under big sur -> boot back to high sierra
Finding a solution I only read the advice to erase the whole disk and reinstall.
Can anyone tell what APFS or container feature(s) macOS adds and if it is possible to remove that feature(s) with a tool like diskutil or similar without erasing?