There are many issues with the proposed Manifest V3, but the relevant part is that extensions will no longer be able to have full control over requests, so ad/tracking blockers will be less efficient at their job. From EFF:

> As the name suggests, the new declarativeNetRequest API is declarative. Today, extensions can intercept every request that a web page makes, and decide what to do with each one on the fly. But a declarative API requires developers to define what their extension will do with specific requests ahead of time, choosing from a limited set of rules implemented by the browser. Gone is the ability to run sophisticated functions that decide what to do with each individual request. If your extension needs to process requests in a way that isn’t covered by the existing rules, you just can’t do it.

https://www.eff.org/deeplinks/2021/12/googles-manifest-v3-st...