This is more a request than a bug report, newer kernels support btrfs raid1c3 (3 copies) and raid1c4 (4 copies), currently a pool converted to raid5 or raid6 by using the GUI options will use raid1 for metadata (and rightly so since it's not recommended to use raid5/6 for metadata), the problem for a raid6 pool is that redundancy won't be the same for data and metadata as warned in the log:
kernel: BTRFS warning (device sdb1): balance: metadata profile raid1 has lower redundancy than data profile raid6
i.e., data pool data chunks can support two missing devices but metadata can only support one missing device, with the new raid1 profiles this doesn't need to happen anymore, so when converting a pool to raid6 metadata should be convert to raid1c3.
Note that if the user downgrades to an older Unraid release the pool won't mount, but it can always be converted before downgrading.
P.S. you could also add convert options to raid1c3/c4 for data with 3/4 devices, not sure that would be used by many, but always nice to have the option if not too much trouble.