SUSE grub has patches to "mount" this subvolume at boot time and the
grub configuration file as generated by SUSE will include commands to
do it.

 
Yes: https://build.opensuse.org/package/view_file/openSUSE:Factory/grub2/grub2-btrfs-06-subvol-mount.patch?expand=1

On top of that, with the secure boot compatible installation (as done by
shim-install), the directory is not actually used or needed anymore. The
prebuilt and signed grub.efi contains all modules.

Thanks very much Fabian  for those details.

    https://en.opensuse.org/SDB:BTRFS

where it is talking about those 2 subvolumes it says

    A rollback of the boot loader configuration is not supported.

Was that because, in the past the grub.efi did NOT contain those modules and now that it does that restriction no longer applies ?

Does this mean that in the future TW will no longer create i386-pc and x86_64-efi subvolumes ?

Can we safely remove those 2 subvolumes in existing installations or is there a plan to clean that up since they are not needed anymore?

It looks like the x86_64-efi files are now located in /usr/share/grub2/x86_64-efi now ?

Thanks very much in advance!