I've heard it explained that "hey" used to be more of an urgent way to get someone's attention, rather than a casual "hello" like it is now, so it sounded rude to some older folks.
I remember my mom getting uptight over the word "sucks", as in "that sucks" or "it really sucked". Literally everyone was saying it, there was no way I could help it lol
A few years ago a very boomer gen-xer tried this on me and got very enraged when I would say "hey" instead of "hello {his name}". At one point even threatened me.
This reminds me of that old joke "what do gay horses eat? Hay hay haaaayyyyyy" that was told to us by a teacher in I dunno, middle school probably. Gotta love the 90s
"Hej," pronounced "hey" is Swedish for "hello." Also "Hej hej" these days if you want to be more casual. It seemed weird to me at first, like "Hej mormor," for "Hello, grandmother," seemed informal, but if I said, "God afton," (good afternoon) my cousins said I sounded like a government issued language tape.