i assumed the mario items were permanant since they're in the promotion section of the nook shopping app
i could be wrong but yeah, i'm pretty sure they won't go
They're probably indefinite at the very least. Maybe they'll be gone in, say, 5 years from now, but it's safe to assume they'll be around for a while for sure.
Luckily they are not time limited!
Thank gourd for that because:
1) time limited stuff stresses me out
2) we've got all these other cool items to order that ARE time limited now (5 per day is just bogus)
and 3) I'm super indecisive and don't know what I'm doing with them yet, other than the fact that I do indeed want to use them but don't want them to take up space in my already cramped storage!
I don't think it matters much. Once you buy them/catalog them from someone, you can just order them via Nook Shopping...ie. there will never be a point in time where these items will be hard to find.
Once you've bought them from the promotions tab they are catalogued so you can order them from the nook shopping app or terminal same as regular items. I confirmed this with the Wario outfit
So I guess even if they removed them from the promotions tab, if you've previously ordered them, and need more, you can order them from nook shopping same as any other item (that's purchasable).
It they were meant to be time limited, they'd likely would have been added to the same place you buy the limited stuff, like the Pi Pie.
All the past Animal Crossing games have had Mario furniture. Even HHD celebrated Mario 30th Anniversary. I would say they are here to stay permanently.