ar:rural looks to have very few usagehttps://taginfo.openstreetmap.org/search?q=maxspeed%3Dar%3Arural

[country code]:[where] is used for implicit speed limit. But it looks like to me that not used for AR, eg. comparing with AT.https://wiki.openstreetmap.org/wiki/Key:maxspeed

It rather looks like to me that AR is on the maxspeed per highway type schema ?

Yeah, the urban one is more common (https://taginfo.openstreetmap.org/search?q=maxspeed%3Dar%3Aurban) and is what triggered my PR, as osmose kept giving false-positives and in my PR over there it was suggested to also adapt it here, seeosmose-qa/osmose-backend#2635 :)