I support such a tag.
If there is no speed limit sign, surveyors should be able to enter that fact. I can survey that there is no sign a lot more easily than parsing local legislation in my mind to determine the legally allowed speed. There is an obvious analogy to maxheight:signed under bridges: obviously there is a max height, but if it is not signed, it’s a lot easier to tag that fact than to remember what is the locally legislated maximum vehicle height.
maxspeed:signed=no was my thought by obvious analogy to maxweight:signed or maxheight:signed, but I won’t be opposed to speed_limit_implied=yes
maxspeed:signed | Keys | OpenStreetMap Taginfo is barely used, so it is good to ask before a major editor starts inserting it widely