The logical screen size was the same as the iPhone 11 Pro (375x812), so the iPhone mini didn’t add a new display size, and everything rendered exactly the same as on the 11 Pro (which itself was merely a vertical extension of the earlier iPhone Plus resolution). The display is rendered internally at the 11 Pro’s physical resolution (1125x2436) and then downscaled to the mini’s physical resolution (1080x2340). Similar internal downscaling had already been done on the earlier iPhone Plus models, so also nothing new. The 11 Pro is still supported by current iOS, same as the mini.

It was, to the contrary, the non-mini iPhone 12 that introduced a new resolution and required adjustments.

https://iosref.com/res

> on the software side it must have been at least a non-zero pain to deal with

Compared to the iPhone Duo now in the lineup? And the iPhone Air was announced with some pride from the Apple hardware team for its special miniaturized hardware.

There may not be a market for an iPhone Mini-sized device when looking at the buyers, but the custom hardware and tweaks to software aren't a clear consideration for Apple when you look at their behavior with other products.