Yes, btrfs was a tech-preview, meaning use if you want with no support or future guarantees. It has been officially dropped from future plans now.
And they're working on adding CoW and deduplication to XFS.
Wow I didn't know that. Very interesting to hear. Hopefully Docker support is considered as well.
overlayfs + XFS works very well, so it's probably not a priority (wild guess)
XFS went through a tech preview -> support -> default over the RHEL 5/6/7 lifecycles.
The fact btrfs has been yanked from tech preview would tend to suggest feedback has been so bad that they believe it can never make it to being supportable.