Hard to tell, whether a smaller or greater percentage of people alert to their passing intentions, on MUPs.
Myself, over the years I've been a walker, runner, cyclist. One thing I have always done is: approach from behind with a fairly low speed differential while passing. No matter what else might be the case around others, it's almost always the safest option.
I've never been able to determine what someone's likely to do, if announcing my approach, when a single walker or multiple walkers are in a MUP lane. As frequently as not, it seems they're likely to jet left into the "passing" lane, completely unannounced, as though they've lost their minds, gotten scared, and/or utterly forgotten the traffic rules/guidelines. And so, I pass relatively slowly. I might well not announce, if the walker or jogger is solo and in the right-most portion of the right-hand lane. If it's a cyclist I'm passing, I almost always announce.
One thing I wish folks would do: if you're going to be sharing multiple lanes on a MUP or any other roadway, whether it be in a truck or car or bike or walking/jogging, always adhere to the basic traffic rules and never make rapid unannounced maneuvers blindly. Which is about as likely to happen as my finding $1B in gold buried in my back yard.