Probably because its a question of the same general nature as the length of a piece of string. Or when daring becomes reckless.
Even for the same rider, the ”should” probably varies with the type of riding.
Most of my rides are commutes, with decent bail-out opportunities in case limp-home mode shouldn’t work.
There are several repairs I’m happy to try on my commuter that I wouldn’t try on my ”race” bikes. There are also levels of wear that I accept on my commuter that I wouldn’t accept on my ”race” bikes.
So far, repairs I’ve refrained from trying/tires that I scrap are broken beads and tires showing lumps and bulges not associated with an obvious damage. And wide-area damage or wear.
.... can often be repaired and last well enough to let the tire be worn out.
Ignoring tires with extra flat protection, ”sidewall” is merely a location, and not a design element description. The casing/carcass is the same underneath the tread surface as in the sidewall. Only difference is that the rubber outside the casing is thicker.

I’ve never had much luck super glueing cuts. Lack of success made me stop that fairly soon. Liqui-Sole has shown greater success for me, but even those have been possible to scrape out easily, which made me wonder what good they made.
What I’ll use on the inside depends on what I can make stick. And how many patches I need. When I had the studs wear through on the inside it became a double no. The vulcanizing compound wouldn’t stick, and there were many holes. I settled for contact cement and patches cut from a thick inner tube.

Duct tape can be a real friend. I always carry a couple of feet wrapped around the seat post. If applied bead-to-bead so that it overhangs and the bead helps lock it in place, it’ll hold up for even inch-sized cuts on mid-pressure tires.