Where is the Mail plugin on macOS Sonoma or later?

With macOS Sonoma, Apple discontinued Mail plugins and replaced them with a new extensions architecture. Unfortunately, the new extensions are more limited in functionality and no longer allow us to access your email in the way we need to make email archiving work. They also no longer lets us insert our own commands into Mail. A future version of DEVONthink might offer more sophisticated email archiving options that don’t rely on a Mail plugin.