If you want to use btrfs on RHEL8, you still can: go to https://elrepo.org/ and install elrepo-release, then install kernel-ml from their -kernel repo and btrfs-progs from their -testing repo.
Running an unsupported kernel doesn't mean that support won't help you at all. It just means they won't help you with kernel-related issues. There's still a lot of value in support for the rest of the system.