No one here, or from the Community (AFAIK), is actively working on it, so postponed means here not real timeline, and if no one steps up it's unlikely for 6.0 I'd guess.

Some main issues are, fsync is _really_ slow, about an order of magnitude worse as ZFS (an FS with a similar feature set), this showed especially in our installer tests, as dpkg per default does an fsync after each file extracted (this could be work-arounded for the installer case, but stay an issue after installation..). RAID 5/6 is still unstable and replacing failed disks is not too ideal yet either. As soon as this is addressed, there probably will be more interest and motivation to support it officially, but until then we just see it not as ready to make it a "Proxmox VE first class FS" with more integration. I mean its included in the kernel so you can already try and use it as a basic underlying storage.