> I am left having to assume that, in the absence of some clear indicator to the contrary, whoever I am writing to will actually have rather strict expectations

This is self-defeating. You have the option (and I recommend it) to intentionally adopt the opposite assumption:

Zero communication is urgent, unless explicitly described as such.

It might be appropriate to make exceptions for certain people. Parents, partners, children. Maybe some work people during a crunch. Maybe some friends going through difficult times.