A way to tag “there is no speed limit sign on this section of a way, but I’m not sure in which speed limit zone it belongs” is useful. The information value is small, but I see no reason not to have such a tag. The information could be combined with information collected by other mappers such as way sections where the speed limit zone is known, location of speed limit and city limit signs, etc. to logically conclude by an armchair mapper (or even a router) what the speed limit of that “no speed limit sign” way must be. If a certain way without speed limit signs can only be reached through ways with a know speed limit zone, then that way must also be in that speed limit zone. A maxspeed overlay in StreetComplete would nicely visualise it.