The entire meta-point of MV3 is, ostensibly for consumer protection, to prevent extension authors from dynamically executing code that has not been reviewed by

The entire meta-point of MV3 is, ostensibly for consumer protection, to prevent extension authors from dynamically executing code that has not been reviewed by Google’s review process, as well as from executing code synchronously to choose to block a network request. Of course, userscripts are dynamic code not delivered as part of the extension itself. And so are effective ad blockers - when one side of an arms race is fundamentally limited from running code, it becomes open season for the other. The conflict of interest left a foul taste in many people’s mouths.