As roaming data outside the EU is priced to discourage any usage this would not be a sensible use of any resources.
Even if restricted to just the EU there are usually several possible roaming partners in any country, so keeping such a map up to date could be a problem.
Hello having been on a few networks and looking at their coverage maps that state 4g is available only to find these black spots that aren’t shown where you only get 3G or worse
due to hills valleys buildings it not possible to really do what you want as networks coverage are anything but accurate all over the world ,
there is an old saying you don’t always get what you expect
where I live move 1 mile and you go from 4g to nothing