Mail.app in macOS 10.11 and earlier used to check the plugins compatibility using the SupportedPluginCompatibilityUUIDs key in the plugin’s Info.plist. For example a Mail plugin would only be compatible with macOS 10.11.6 if its Info.plist contained the following:
OS X 10.11 ‘El Capitan’ added a new feature to Mail.app Swipe to manage your inbox:
Swipe to manage your inbox.
Now you can take care of your email with a swipe, just like on your iOS devices. Need to triage your inbox? Swipe right to mark an email as read or unread, or swipe left to delete. You’ll be focused on what’s important in no time.
I find this new feature extremely annoying as I keep triggering it by accident. Sadly it seems that there is no preference (even secret preference) to disable this feature. But the good news is that Mail.app supports plugins.