If I understand the second part of this article correctly, the first proposed solution was to replace those magic subvolume directories with auto mounted directories, but some standard uses of btrfs have so many subvolumes, the kernel code managing mounts was running into scaling issues. Which would probably be a good problem to address eventually, but is a big effort in itself.

An ugly workaround would be to embed the subvolume id into the inode number, but it would almost require 64 bit inode numbers and some tools make daring assumptions about the stability of inode numbers across reboots.

As far as I can see, subvolume ids are stable across reboots.

It seems like there is no agreement on if inodes should be unique or not, or what information is required to uniquely identify a file or directory. It does not surprise me that Linux is looking for a solution while this problem is open.