I can’t speak on itch.io’s behalf on this matter, but I’ve seen physical magazines and games sold alongside the digital downloads on several store pages. And I’m pretty sure if I looked hard enough I would find musicians offering/advertising their T-shirts.
Like with many, many things concerning “Is it okay if I do [X] on itch.io?” I’m pretty sure the answer will be “It’s fine, as long as you don’t go overboard.”
As an personal aside: I’ve seen people getting more exited about pins than stickers, although I guess there’s more of a financial risk/investment involved. As well as more expensive packaging and customs officials smashing everything that comes through with their state-issued hammers…
I hope any of this helps.