With some tinkering you can also route arbitrary addresses to plus-aliases (@example.com -> inbox+@example.com, where "[email protected]" would be your "main" address). This correctly sorts incoming mail into folders just like normal plus-alias usage would (if those folders exist), but in your inbox, the mail will still show the original mail-to (*@example.com, not the plus-alias version), and when replying, it'll use the correct address (not the plus-alias).

Support helped a lot when I tried to set this up. They initially said it wasn't possible, then came back a while later with a correction.