It took the form of a tip jar. It was my main source of gas money when a barista.
Tipping at nicer coffee places (i.e. where they offer espresso-based drinks to order) has been a thing for a very, very long time. I can't recall ever not seeing a tip jar and have been going to places like that for probably 25 years, although if you told me the expectation was stronger nowadays that wouldn't surprise me.
The low end places (Tim Horton's up here, I guess Dunkin Donuts would be the comparison), don't expect tips or put out jars / enable them on their terminals, but Tim Horton's has always had a charity jar at the counter anyway so I would think most people would prefer that one.
The Pulp Fiction shake quote always comes to mind for me:
Vincent: "I gotta know what a five dollar shake tastes like"
When did coffee shops in the US start asking for tips? It most be a relatively thing. Never saw tips asked for at coffee shops when visiting until the ipad checkout thing (square?) appeared. Is it a nationwide thing now or just confined to certain regions (coastal metro areas?)?