Earlier I spotted that a couple of relations here used route=road, but indeed now I see that some also used route=street.

If I get this OSRM code correctly then this is specific to some hardcoded US routes not to route=road in general, right?

I see that major roads in Australia are also named as routes, and these should have route relations. In respective guidelines, if I'm not missing something, it also doesn't explicitly discourage the use of route relations for smaller roads, though.

I guess the wiki talk page of respective tag[1] would be the most obvious and convenient place where to comment on how this tag should be used.  Currently route=road wiki page doesn't mention any restictions on which networks (excluding some local network of numbered roads within a city, or local networks in some countries perhaps?) can use this tag nor that the use this tag could somehow dilute the data. If such restictions apply then for me it remains a question what is the alternative for smaller roads (type=street doesn't seem good one due to its documented purpose).

[1] https://wiki.openstreetmap.org/wiki/Talk:Tag:route%3Droad