Oracle UEK kernels have been providing the btrfs filesystem since release 5 (UEK5). The most recent UEK kernel is release 6 (UEK6), which is based on Linux kernel 5.4, where as UEK5 is based on Linux kernel 4.14. UEK6 as you would expect contains a lot more fixes and improvements as compared to UEK5.
With specific regard to the btrfs filesystem, I will skim through a few significant improvements and fixes in UEK6. Bear in mind that UEK6 btrfs includes many other bug fixes other than what is listed below. In general as a rule of thumb, it is encouraged to use UEK6 when possible. Also, the backporting of newly reported btrfs mainline bugs is more likely to occur and more feasible in UEK6 when compared to UEK5.
Supports zlib and zstd compression level, -o compress=zlib:9
Unprivileged ioctls allow users to enumerate subvolumes
rmdir() can delete an empty subvolume
Dedupe syscall implementation does not restrict the range to 16MiB
Improved trim support - Skip unallocated chunks that haven't been touched since the last mount
Space handling - Improved ticket reservations, flushing infrastructure, reduced metadata reservation for updating inodes, global block reserve minimum size (affects small filesystems), overcommit logic with regard to global block reserve
Tree checker - More sanity checks on ROOT_ITEM, EXTENT_ITEM and METADATA_ITEM, Block Ref Item, EXTENT_DATA_REF
Mixed Enospc fixes for high fragmentation and over-reservation
Performance improvements - Replace blocking mode with the spinning mode to improve latency and scalability
Better read time and write checks to catch errors early and before writing data to disk (to detect potential memory corruption on data that get checksummed)