https://btrfs.wiki.kernel.org/index.php/Changelog#By_feature
don't know how much debian backports btrfs fixes (or new features)
changelog here:
https://salsa.debian.org/kernel-team/linux/blob/046a5da6eb748c59f7402d5aae84ab7543dbb396/debian/changelog?expanded=true&viewer=simple
downgrade could be problematic if you use ZSTD as your compression for example, but not limited to this probably.
IMHO - test the downgrade on a separate pc or disk. (and never touch a running system)
at least for those running btrfs this kernel switch should be done with extreme caution.