It was available as a technology preview in RHEL 6 and 7, but dropped in 8. There apparently wasn’t much demand for it, and the reputation of BTRFS isn’t exactly synonymous with the image of reliability Red Hat strives for. There’s also the idea of maintenance and support burdens, and Red Hat themselves have launched their own stab at a fs with an integrated volume manager called Stratisd, though IBM supposedly absorbed the team that was working on it for their own products.