Pretty much every Linux bootloader supports BTRFS these days.

The critical thing though, is that happens if your BTRFS partition gets corrupted? You just lost your failover since both your primary and failover are on the same partition.

That's fine on a desktop system where the user can boot into a recovery image and repair the filesystem, but it's not fine when you do a completely automated system upgrade. So for a kiosk, console, or other embedded system, the two partition setup is more reliable than a BTRFS root with subvolumes.