This is slightly in the context of my testing the Thunderbird beta of the same version (8.0b2) in parallel, where it has done odd things with these settings in import, but I shall avoid those details here (in this context, it did not import the top/show/class settings at all, but I think I had a K9/TB version mismatch at the time, not both 8.0b2).
Essentially, I’m not sure what is supposed to be being used instead of folder classes (and couldn’t find anything obvious online). While it all currently seems to be working, albeit mostly without the folder classes, I am getting the impression that the folder classes are still being used under the hood.
E.g., if I am in an email and select Move, I now only see what are/were my first class folders (used to be the lot, I think, but with 1st class at the top). The kebab menu offers Show hidden folders and I then see the rest fine (although that’s a bit annoying as first-class are generally monitor folders and the rest are filing, so in my workflow Move is almost always going to need that extra menu step).
Regardless of that, if I go to Manage folders to now see how to change them, I do not see anything about the old second class folders being “hidden”. There is an option Show folder which may be intended as “not hidden” but it’s poorly named if so.
There is also Show in top group which I’ve not noticed before and it looks as if that might be trying to be an equivalent to first-class folders, but I am only having to guess at what that menu above meant by “hidden”.
Maybe “hidden” refers to all folders that are both not “top” and not shown?
The other discrepancy, which makes me think class is still there behind the scenes is that under account settings / notifications, the Notification folders setting still only has options pertaining to first and second class folders, not the two categories above — but of course, it is now impossible to change a folder’s class to affect notifications.
Sorry if there’s a forum for beta users where all this has been laid out! All I saw was the announcement of folder class removal but with no extra detail.