1. It is a niche feature to Mail rules that isn't exactly marketable to the average user, but instead exists for power users that seek it out. Like many small features and secrets in macOS, it is more often discovered than known out of the box.

2. It is pretty obvious from selecting the rule when it prompts you to open a script in Finder. Anyone interested in learning AppleScript can search it online or in their documentation, they're not going to distill the full scope and usage for AppleScript for every corner of the operating system where it can be utilized.

I also hope Shortcuts continues to include the AppleScript action, since Automator will soon be gutted and regular users won't know how to invoke osascript from the command line. There are still occasional additions to the AppleScript dictionary, a few last year even for the iWork suite.