Will Shields still work after Manifest V3?
Yes, they will! Brave has long been prepared for this and has their ad-blocking built into the browser through Rust. As a result, Manifest V3 will have absolutely no effect on Brave.
To better quote from what has been said by Brave directly about whether Manifest V3 would break Shields:
This Manifest V3 been delayed for a couple years. They first announced it in 2019 and our team put together a rewrite of our adblocking engine with Rust and then put it into the core for the browser for the network request. So we’re not coming in on the same API that’s affected by Manifest V3. Brendan has also talked about possibly providing support for other adblocking extensions too, but that will be disclosed at a later date. When this rolls out, most likely you’ll see a softer experience on blockers that aren’t on Brave, but Brave will still have the blocking you expect.