Mail app in macOS had support for plug-in and many professional users were relying on these plug-ins to manage their incoming and outgoing mail.
Apple introduced MailKit allowing developers to build Mail extensions as a replacement for legacy plug-ins. As of now, several projects are still using plug-ins. These plug-ins will stop working with update to macOS 14 Sonoma.
Some developers already started the migration to Mail extensions.
Apple Developer: MailKit