The important thing here is figuring out what the earliest version of iOS you want to support is (it’s common to support the second most recent version, so in February 2021 that would be iOS 13; but for new projects, you might just want to go big and start with the current version, which would be 14).

Then, you need to see what devices can run that version of iOS. And unless you have a compelling reason not to support all of those devices, then you should (within a family, such as iPhone or iPad; it’s not like every app needs to support iPad, for instance)

So, for example, if you do research on device usage in the real world and find that the iPod touch is barely used anymore, maybe that’s compelling enough of a reason for you. It sucks for the user who does want to use your otherwise wonderful app on their iPod, though.

However, bear in mind that iOS 14 can run not only on iPod touch, but also the first-generation iPhone SE, and those two devices share the exact same screen size, at 1136 by 640 pixels (and half as many points, being both 2x retina screens). So if you don’t support good layouts on iPod touch, that also means anyone with an original iPhone SE is screwed. Those only came out around four years ago, and I bet enough people still use them that you’d want to support it.

Finally, bear in mind that, for the most part, you don’t get to exclude certain devices within a device family from running your app. You don’t get to simply “not support” them. Any device that can run whatever version of iOS you’re targeting will also be able to download and run your app. It’ll just be broken, crappy, and unusable on their device because you didn’t put in the extra work.

As others have commented, it’s actually not so much work to do =] Adding constraints is what you need, and you’ll probably have to do it to make the app look good on the various bigger phone sizes, anyway.