I have a secondary computer that can't be upgraded beyond High Sierra, just as I have a still older MacBook left running "Snow Leopard," which some people were supremely attached to but which in my experience got to the point of "it can still run this old program, but seemingly nothing newer..." Anyway, it's been long enough since High Sierra came out that I don't remember a lot of indignant "never upgrade under any circumstances ever" warnings back then.