Many more Settings in MC21 were automatically migrated over from MC20 (and probably earlier versions), but not the Library Manager loaded Libraries. However, this setting (which Libraries are loaded into MC's Library Manager) is included in the settings portion of a
Library Backup.
If you just backup
a Library in MC20,
any Library, in MC20, and then restore the Settings in MC21, it will migrate those settings over. The Library format itself has not changed, so they'll load fine. You don't have to restore each individual one, just access them right where they were before.
If they ever do change the Library format, which
could happen but has not thus far, then MC21 will automatically upgrade them (and then they'll no longer be backwards compatible with MC20). But, for now, that isn't even an issue, and MC20 will still happily open the older Library (and MC19 as well, it has been a while since they changed the format).