I got it from this comment: (
https://news.ycombinator.com/item?id=32755925); and Chromium's official blog that hints at that:
https://blog.chromium.org/2020/12/manifest-v3-now-available-...
> To give users greater visibility and control over how extensions use and share their data, we’re moving to an extensions model that makes more permissions optional and allows users to withhold sensitive permissions at install time. Long-term, extension developers should expect users to opt in or out of permissions at any time.
> For extensions that currently require passive access to web activity, we’re introducing and continuing to iterate on new functionality that allows developers to deliver these use cases while preserving user privacy. For example, our new declarativeNetRequest API is designed to be a privacy-preserving method for extensions to block network requests without needing access to sensitive data.
> The declarativeNetRequest API is an example of how Chrome is working to enable extensions, including ad blockers, to continue delivering their core functionality without requiring the extension to have access to potentially sensitive user data. This will allow many of the powerful extensions in our ecosystem to continue to provide a seamless user experience while still respecting user privacy.
Basically, Chrome is hoping to use the same psychological effect as Apple's "Ad Tracking Transparency" (the opt-in screen for cross-app tracking) to make people opt-in for "read/write data on all sites" permission, to try to transition the amount of adblockers with that permission from 100% (currently) to ~20% (the amount of people who opt-in to tracking on iOS) by scaring users about extension permissions.