iOS 12 officially supports the following devices, per Apple.
iPhones: XS, XS Max, XR, X, 8, 8 Plus, 7, 7 Plus, 6S, 6S Plus, 6, 6 Plus, SE, 5S
iPads: 12.9" Pro 2nd gen, 12.9" Pro 1st gen, 10.5" Pro, 9.7" Pro, 6th gen, 5th gen, Air 2, Air, mini 4, mini 3, mini 2
iPods: touch 6th gen
That's 26 different base models, of I don't care to count how many different physical form factors, each of which was/is offered in multiple configurations and has different onboard hardware and capabilities.
Incidentally:
I don't understand why they can't write their OS updates to work properly on a few generations of older models
They support 8 generations of iPhones, 11 generations of iPads, and one generation of iPod, with the oldest supported devices dating to 2013.
If your clients do not care about security updates for older models, and happily buy new models which carry a 300+% profit margin, why on earth would they justify not doing what they are doing. It is money, plain and simple.
And this minuscule fine will only make it worse. Because now they know the low cost of continuing to do that.