Soi Chana Songkhram is a good example of a local street in Bangkok that I think should be mapped as highway=service + service=alley.
More than just being single lane, it would be impossible for two cars to pass each other, even if they (illegally) used the sidewalk space. Buildings and property objects (signs, plants) simply do not allow this.
I looked around a bit and it was very difficult to find other local streets that were that narrow, the vast majority of local streets allow two cars to pass through. So I ran a query on Overpass and found a number of roads currently mapped as alleys. So I used whatever tools I could to take a look at them, and most of them seemed correct: they were very narrow and it was absolutely impossible for two cars to pass each other. A few of them were so narrow that only motorcycles could pass through. A few of them should be other types of service=*, such as parking aisles and driveways. So I think, at least in Bangkok, the current practice matches the wording of service=alley since 2013: “the very narrow streets which run in-between buildings, providing public through-access”. In the case of Soi Chana Songkhram and the other alleys I found there, mapping them as alleys renders nicely in all styles on the main site and the routing works correctly with all 3 routers on the main site (OSRM, GraphHopper and Valhalla) with all the 3 main modes of transport (car, bicycle and pedestrian).