Anecdotally I know that none of my neighbors are mortgage free. However I also have a few friends that had major cash windfalls things like startup exits or time working in hedge funds, each of them owns 3-5 houses outright as investments.
So, with this small sample I can easily see that of the total number of homes owned by friends/neighbors, 40% being mortgage free sounds about right, but far less of my friends/neighbors are mortgage free.
But if you are moving right now for any reason, you aren't likely to be taking on more debt in the process. There's a pretty strong incentive to do what it takes to get debt to or near zero.
In California, the haves will have houses. The have nots will have ADUs.
Our world is structuring itself to (more) obviously reveal the wealth disparity.