Assorted tidbits that may or may not be of use to those following this thread:
1. As of August 5, I was still able to save whatever emails I wanted to select from any Mail mailbox to the AOL "Saved" mailbox with no errors or disappearance problems of any kind whatsoever.
2. As of approximately one month ago, I could with Mail.app also move any email I wanted from the AOL IMAP inbox to the AOL "Spam" mailbox. This is what some official AOL help documents indicate is the equivalent to using the AOL client app "Report Spam" button, so I assume this is, or at least was previously, officially supported with IMAP access to AOL email as the approved way to report spam when not using AOL client or web mail access.
2.a. At some point in the last ten days or so, I noticed this 'report spam' equivalent was not working properly with Mail.app. I don't know the exact date this started happening since I don't often have to use the feature, but what I do know is that as of ~ July 28, I started getting error messages similar to the ones we now see for moving messages to "Saved." So far, the error messages are all that happens with this -- IOW, there is no disappearance problem, only a refusal to accept any email into the "Spam" server folder via Mail.app.
3. In Mail.app, using the menu "Copy to..." option for attempts to move messages into the affected AOL-based folders works very much the same as with drag & drop, except the existing copy is never deleted from the source mailbox, so I suggest this is a better way to test for the problem than by risking any messages you might want to save.
3.a. Using the copy option may initially look like it succeeds, even when it doesn't. In my case, messages copied to "Saved" sometimes did appear there in Mail's view even after opening the "Saved" mailbox view, which I assume from Mail's Activity window does cause some small amount of syncing. However, within 10 seconds to about a minute, the newly copied message still disappears, which I assume is due to some server-side problem a 'full' sync shows.
4. Until AOL sorts out all the details of its "new" incarnation, It is probably best not to count on saving anything to its servers, even with the web mail interface.