According to OSM Wiki's article for maxspeed=, it has been common in a number of countries to add a "XX:rural" value there and not in source:maxspeed. If you feel that this is inappropriate, then I leave it to you to do an Overpass API query of all instances and change them.

If I added a specific number to a maxspeed= tag inside Poland, you can be sure that this was based on personally seeing the speed-limit sign; I did all such mapping on my bicycle.