I think you may have been confused about the Manifest V3 API changes, which were controversial because they didn't support every feature of the old API. The mainstream ad blockers all wrote new versions for Manifest V3.
It is widely known that Manifest V3 reduces extensions ability to perform SoTA ad blocking. It limits heuristic based filtering, under a guise of privacy.
It was more of a security related change. MV3 overall objectively is far better for browser security than MV2. MV2 was essentially giving extensions a full on free RCE pathway. MV3 is what it should’ve been from the start imo.
MV3 still allows you to run content scripts, which can inject any javascript into any webpage. From there, you can do anything you want. You can steal passwords, tokens, show popups, redirect, ... etc. Preventing extensions from dynamically modifying network requests doesn't change that.
Manifest 3 explicitly enables ad blocking through the declarativeNetRequest API. It's trivial to do so, and many blockers exist in the Chrome Web Store.
They have an API basically dedicated to this: https://developer.chrome.com/docs/extensions/reference/api/d...
I think you may have been confused about the Manifest V3 API changes, which were controversial because they didn't support every feature of the old API. The mainstream ad blockers all wrote new versions for Manifest V3.