From the source code [1] it would appear that ACL for BTRFS is categorically disabled across all branches and targets.
CONFIG_BTRFS_FS_POSIX_ACL=n
If that is due kernel size concerns then I would appreciate if a developer could be obliged to enable/implement ACL for BTRFS in Master & 19.07 for the mvebu target since that target class should be able to cope with the potential increase/cost of the kernel size.