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.
Image Transcription: Twitter Post
The Pumpkin Dipshit, @ParSpec
Remembering how common it was in like the early nineties for a kid to say "hey" and some old timer to respond "hay is for horses!" as part of some campaign to label "hey" as rude and force kids not to say it. They're gone now and we still say hey. You lost, old timers.