Aggresive rests
Some background: I created a score with 64 measures, but when I entered notes in the first 28, MuseScore converted the remaining measures to one measure with a 36 rest duration. I tried breaking the 36 rests, but MuseScore didn't allow me to select the measure, it only highlighted the "36" and the measure bar. 
Comments
They already exist, but they do not manifest as presumed as how you'd like. See the handbook here:
https://musescore.org/en/handbook/3/measure-rests#multimeasure-rest
In reply to They already exists, but… by worldwideweary
Thank you!!
In reply to Thank you!! by michaelcalo34
Note that multimeasure rests are off by default*. If you are seeing them, you probably have accidentally pressed the keyboard shortcut M which toggles them on and off.
*Qualification of that. They are off when you first create a score but are on by default in a part that is extracted from that score. This is as it should be as the conductor's score should normally not require multimeasure rests, but individual parts will need them when nothing is happening in the part.