In slowly getting through my trip report, I note that in Bilbao shopkeepers greeted me, whom they hadn't seen before, with "Hola" rather than "Buenos Dias" or tardes, etc. Is it correct that this wouldn't have happened a few decades ago, and is a sign of reduced formality? Is it similar in Latin America, Mexico in particular? It isn't unique to Spanish; "Hi" is said a lot in the U.S.

in Spain, we use Hola as informal. It is becoming more and more use rather than just said Buenos dias etc.
Modern times tend to give folks more freedom to interact quicker. Yes, that would not happened in the 1970's Spain that I lived in.But times change for the better...