The Btrfs "Subpage" functionality is for handling sector sizes that are smaller than the page size, rather than traditionally the two sizes needing to be the same.
Btrfs has a lot of code around its sub-page support and needed special handling for RAID5/RAID6. That code has been under review and appears poised for introduction in Linux 5.19.
As of this past week the RAID5/6 sub-page support is ready and enabled within kdave's for-next branch ahead of the Linux 5.19 merge window opening later this month.
Enabling the RAID5/RAID6 sub-page support for Btrfs was carried out by SUSE's Qu Wenruo.
More broadly, the current state and information around Btrfs' native RAID support can be found via the Btrfs Wiki . Specifically there is also the RAID 5/6 page that does still carry the warning, " The parity RAID code has a specific issue with regard to data integrity. It should not be used for metadata. For data, it should be safe as long as a scrub is run immediately after any unclean shutdown. "