In principle an AV extension could read the scripts on a page and either analyze them there (or background web worker or something) or (if the APIs allow) pass

In principle an AV extension could read the scripts on a page and either analyze them there (or background web worker or something) or (if the APIs allow) pass them off to the native AV process for examination; it could theoretically be useful to go beyond simple blocklists and try to identify badly behaved or malicious scripts before they actually get executed in the users browser. Unfortunately, AV vendors have not really demonstrated the ability to do anything like that in a reliable or secure way.