I can break the /etc/fstab into separate files in a directory, but
/etc/fstab.d won't be read by default.
I would have to use an explicit argument "mount -T /etc/fstab.d" but the
-T (and the equivalent --fstab) are only allowed to be used by root.

I like the idea of splitting the fstab into files so I can
compartmentalize the information and manage the files independently in GIT.
When I install a new system I'd just plop the files into /etc/fstab.d
and not have to use hacks like appending or patching, which make the
contents of the file look like junk.

Could you please allow "-T /etc/fstab.d" to be work for users, and just
exclude users from using other path-arguments?
I suppose this would have to be implemented upstream of Debian, but I
don't know how to file that.

References:

https://askubuntu.com/questions/168290/why-cant-mount-read-files-in-etc-fstab-d

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=663623

https://marc.info/?l=util-linux-ng&m=132740311801201&w=2