Apple’s MailKit framework, introduced in macOS Monterey in 2021, allowing developers to create modern and secure extensions for the Mail app on the Mac. These extensions can block content, perform actions on messages, enhance security, and more. However, this also meant legacy Mail app plug-ins weren't going to work with a future macOS release. And it seems that time has come.
AltStore confirmed with Apple at WWDC last week that macOS Sonoma, which will launch later this year, does not support plug-ins and only supports MailKit-based extensions. MailKit extensions are more secure than plug-ins. However, they also have less functionality. So, some plug-ins may not be able to become extensions. First beta of the Mail app in macOS Sonoma does not allow legacy plug-ins to run already.