If I understand the second part of this article correctly, the first proposed solution was to replace those magic subvolume directories with auto mounted direct

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.