The 'making extensions safer for everyone' Manifest V3 keeps the ability to spy on all traffic, but removes ability to modify headers/block requests. Killing Blocking (blocking = synchronous meaning wont complete until extension decides what to do with it) webRequest API means no more:
-intelligent/dynamic blockers
-User Agent modifications
-modifying CSP rules to disable javascript/web fonts etc
Google is removing control from users hands, from the code users want to run. This is one of the steps in a battle against general computing. https://boingboing.net/2012/01/10/lockdown.html You no longer own a computer, you own a Google Web Appliance.