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With manifest v2, extensions could block the content however they wanted, reading and modifying DOM as they see fit.
Google claims that it is a security risk, so with manifest v3, extensions can only create and give the browser rules and the browser itself will block content based on them. The rules have a limit in size and capabilities.
If that was still not clear, try thinking of unrestricted SQL access vs a UI for modifying a database.
Some Chromium browsers like Brave and Vivaldi already announced they’ll extend it for as long as they can, and when they no longer can’t, they’ll think of something else like improve their own blockers.