That's very strange that it doesn't show up there. Even OSX is regarding it as a valid plugin but After Effects refuses to recognize it and you mentioned it happening to a previous version of After Effects as well...
Just one more thing to make sure:
on that .plugin file you should be able to right-click>Show package contents to browse the contents of the plugin archive. Within it, verify that the contents look like this:
Particularly that the "PixSort" file is within the MacOS folder as it might imply that you have an older download of the plugin or the initial extraction did not work correctly.
I'm booting up my OSX installs and I can't seem to be able to recreate this issue at all to find out what is going on here exactly unfortunately... Nothing seems to stand out to make it seem to be a locale or filesystem issue either from some of the other tests that I've done and the current upload at the moment is using the latest SDK from Adobe(Oct 2018) and it is using a statically linked OSX that supports version 10.11 or above so it should show up just fine... I can offer a full refund to your stripe purchase(ending in "xqJrF6") if it is not suiting your needs at the moment as I can't think of any other reason for it to not show up... Really sorry about all this
