I've submitted feedback already, but I'll put my own observations here too.
I think the release documentation ought to explain why and when "opting in" is necessary. I had many of the same confusions as Kent when I experimented with the new feature.
I think it should be clarified that anyone can use the button to visually see the whitespaces. For some projects (such as one I'm working with) that may be all that's needed because we already have the ability to keyboard any whitespaces we need. It's the ability to insert new whitespaces via PT that's an opt-in feature.
Thanks to John for his helpful answer. I had been questioning why this was a breaking feature, but I do remember that 9.4 and before deleted certain whitespaces, so I see how that wouldn't be compatible with this new system. (Obviously, the whitespaces that our teams need aren't the ones that get deleted, so I had forgotten about that behavior.)