I’ve always used high beams as a thank you/I’m letting you pass/bro, your headlights are off, depending on the context.

Here in Germany it’s common to quickly flash your hazards when you’re on the highway and enter a traffic jam to signal the person behind you “Watch out, I’m slowing down and won’t accelerate again”

We started doing so here in Singapore. Even our public busses flash hazard lights twice to convey a thank you sometimes now. In the context of someone giving way to you.

But on a highway when there’s a sudden slowdown in front, we turn on the hazard lights to convey “dude slow down the dude in front is being weird”. Especially useful when there’s torrential downpours.