Btrfs is a modern copy-on-write file system, present in the mainline linux kernel since 2009, present as an installation storage option on some of the widely used Linux distros, and even default on some of them.
Since cPanel only suffers from the inability to properly compile the Net-SSLeay Perl module on btrfs, it is only a step away from supporting this file system by design.
Moderator note: This request was marked as complete when we removed the requirement that was preventing Net-SSLeay from being compiled properly. This did not add full support for btrfs, however. If you would like to see quota support on btrfs, please vote here: https://features.cpanel.net/topic/btrfs-quota-support