I just last week deployed Btrfs file system on all my backup servers in production. I love it as it runs without any problems so far; after testing it for two years I decided it's time. But I am reading that Red HAT deprecated it in 7.4 and I am bit worried about what Debian is going to do.
I'm not asking if it is going to be the default file system; I would not like that, but is it going to be supported at all?
It is too early to say. If that happens, it'll happen when it happens. If you want influence over the discussion, join the mailing lists and get involved. That's the beauty of a community-driven distribution.
But to try and give you some solace,
There are many companies involved in the development of Btrfs.
Red Hat (through RHEL) only targets enterprise deployments. Many of the other contributors target wider userbases.
RHEL "support" is somewhat different from community support. You're paying them to work through your problems whereas there's really no warranted service provision from the community. We volunteer. RH clearly doesn't think it makes commercial sense —for them— to support Btrfs.
From what we can see externally, Red Hat are targeting a different next-gen storage solution. Why? Maybe they think it fits their customers better. Maybe it's cheaper for them to support. Probably both.
I don't think Btrfs is going anywhere until something comes out that's objectively better. That doesn't happen overnight.