Unless it's just these particular third party plugins (I'm pretty sure I said "third party" in the description), I tried installing an 8-bit plugin called Plogue, and the Mac app Tonebridge Guitar Effects recently got the ability to just come up in Garageband as a plugin (rather than to have to run the app separately), and both worked great in Sierra.
I'd say you might be onto something, except that I did a full clean install on my system - both Plogue and Tonebridge install and run as individual apps, but when I try to get them working as Audio Units in GB or Logic Pro, they get that exclamation point over their names, *even though they come up in the AU list*...
It's really weird.