Thank you @dokuro and @Atomatth, riffing on what’s already here:
Forcing a copy of the background as the square art is an option, but I agree that’s a non-optimal work around and just adds more data. Unselecting does still fall back on the background art, so am using that to my advantage.
I’ve run @dokuro’s initial delete all square art script, and have written something here that deletes/unselects square art from the most recently added item (regardless of library type):
from plexapi.server import PlexServer
def delete_squareArt(item):
print(f'SquareArt Unselected for:\n {item.title} ({item.year})')
item.deleteSquareArt()
latest_media = None #initial definition for 'latest media'
for library in plex.library.sections():
if library.type == 'movie' or library.type == 'show':
for item in library.all(): #select all libraries matched
if latest_media is None or item.addedAt > latest_media.addedAt:
latest_media = item #redefine 'latest media' to most recent item
if latest_media:
delete_squareArt(latest_media) #close loop, delete that item's square art
After adding a new item, am running this to unselect/delete its’ square art. Which is also not heaps optimal, but for now it’s working well enough for me.
Hopefully I’ll find/make a way to automatically run this script whenever a new media item is detected in the library at all and save me manually doing it. Better yet: hoping they add an unselect option like there is for logos.
Also, in case I miss doing that for an item, or if multiple items are added at the same time and the previous code doesn’t wipe multiple items with the same timestamp for you (it does for me but yknow, always expecting it might error for some):
Here’s a variation to remove square art from the variable recent [num] of entries:
from plexapi.server import PlexServer
def delete_squareArt(item):
item.deleteSquareArt()
def delete_recent_items(plex, num_items):
latest_media = [] #initial definitions
for library in plex.library.sections():
if library.type == 'movie' or library.type == 'show':
for item in library.all():
latest_media.append(item) #lists all items
latest_media.sort(key=lambda x: x.addedAt, reverse=True) #sorts list by newest
latest_media = latest_media[:num_items] #limits list to number variable
print(f'SquareArt Unselected for:')
for item in latest_media:
print(f' {item.title} ({item.year})')
for item in latest_media:
delete_squareArt(item)
delete_recent_items(plex, 5) #variable [num] (in this case 5), value for how many recent item entries
#so in this case, the 5 most recent items' square arts are unselected/deleted
#change this number to however many items are needed/wanted
(and of course, can add your
if __name__ == '__main__':
plex = PlexServer('your plex server')
if need be).
Hopefully these are helpful to y’all too~ 
EDIT:
Just learnt that Tautulli can autorun python scripts upon triggers. I’ve not used this before, but I’m adding my first script above to trigger upon a new item and will test if that works to automatically delete/unselect square art. But in theory:
Tautilli’s notification agent → new notification → script → add .py file of code → trigger on ‘recently added’ → hopefully automatic square art removal for the most recent item 
If you’re never wanting any square art, could use that to trigger the delete all script, or the [num] script to make sure the last [num] amount of entries have their art removed on trigger.
Sorry this ended up being a long post/reply for my first post! I’m new here and the square art fiasco was annoying me enough to jump in and do something 
should be fixed in the next Beta 1.43.1.
and what I had picked for the square art.
I would assume that it would make a copy. If you want to explore using the script above, maybe add some breaks or pauses and monitor your local appdata directory to see how much bigger it gets. I don’t think it should be much more but I can’t assume to know which images you choose for your backgrounds.