Since version 23.7-1, anaconda revert "allow /boot on btrfs subvol or filesystem" because of grubby bug. But grubby got a update allowing /boot on btrfs now but not anaconda. Grubby update: Changelog * Sat Mar 03 2018 Nathaniel McCallum <npmccallum> - 8.40-10 - Add support for /boot on btrfs subvolumes
*** This bug has been marked as a duplicate of bug 1418336 ***