There were no changes required for macOS Monterey, only for Sonoma. You can keep using the same Apple Mail plug-in on Monterey, if you want, and that’s what we recommend.
The Check inboxes for new messages not sent to Mail extension feature can be slow on Monterey if you have lots of messages in your inbox, due to a performance bug in Mail. We’ve worked around that on Ventura and Sonoma, but not on Monterey. Mail’s extension support on that version is more rough, and it’s better to just use the plug-in, anyway.