Bottom bracket maintenance - is it a thing?
To the bearings themselves, not really. It's possible to pull the
seals and clean/re-grease the bearings while they're still
installed, but this isn't something it's expected anyone's going to
actually do. (The one you have I believe has bearing seals that are
pretty easy to get to, making this easier to do if you want it.
Shimano Hollowtech BBs have the bearings covered by a plastic "top
hat" seal that's not intended to be removed and re-installed, and
usually can't be without ruining it.)

For all external cranks, it's pretty common to encounter noises that
can only be remedied by cleaning and greasing the spindle interface.
That might be considered regular maintenance.
No.
In general, all cartridge bearings with rubber seals can have the seals carefully removed, which is something of a skill in and of itself, be cleaned with solvent, rinse, blown dry with air, and re-lubricated. The nature of external BBs is that you won't be able to remove one side's seal, which makes the process more difficult but not impossible. As mentioned above, many external BBs have difficult or impossible to remove parts in the way of the bearing cartridges themselves, and doing anything with the bearing isn't really practical if this is the case.


Another thing that bears mentioning in this conversation is that many Hollowtech 2 compatible external cups can have their cartridge bearings driven out and replaced. Enduro and Phil both make presses just for this. What replacement bearing and seals are appropriate depend on the BB.