I used (and loved) Mail Act-On for many years when it was available as a separate tool. Then I used Mail Act-On from MailSuite 2019 (as the only part of the total package) when MailSuite 2019 was available for a one-time payment. Now I have upgraded to macOS 12 (Monterey) and MailSuite 2019 no longer works. I would have had to subscribe to MailSuite 2021 instead for an initial one-time $80 and then $45 annually (or whatever the developer would have charged in a year or so). That is too much for just Mail Act-On.
I switched to MsgFiler instead for a one-time payment of $10. MsgFiler does for me exactly what Mail Act-On did - moving emails into folders. It looks a little less elegant than Mail Act-On and has to be set up and configured in a fairly roundabout way, but it now works just as well for me as Mail Act-On.
Unlike Mail Act-On, MsgFiler is not a mail plugin, but a separate application. However, it is possible to hide it from the Dock and use it in exactly the same way as Mail Act-On (call it with keyboard shortcut - start typing a folder name - move the selected mail to that folder with Return).
Too bad Mail Act-On is no longer available separately and for a reasonable one-time payment. Good that I found an alternative that works for me.