There are already non-self-driving cars that get speed limits from signs. I’ve seen that feature in a Honda for example. I imagine you’d have multiple sources like a max speed for that type of road as a fallback. And you need to read speed limit signage due to temporary limits. There’s also variable speed limit roads near me now and so you have to read those electronic signs unless the database is updated very often (though no humans seem to obey those limits).

No, many ordinary cars contain image classifiers that read speed limit signs dynamically. This is pretty standard. And they sometimes get it wrong e.g. my car reads 80 MPH as 60 MPH about a third of the time, much to my dismay.

The dynamic classification is required because the world isn't static. An increasing number of locales have digital speed limit signs that vary the speed limit dynamically, some times independently per lane. Automation requires cars to respond to the world as it is, not how the world was when it recorded a month ago.

then it's not really (self)driving.

who maintains that worldwide dataset?

Why would they?