Currently, uBlock Origin and other ad blockers rely on features enabled by the webRequestBlocking permission, which lets them block any request that is only useful for ads or tracking. Manifest V3 hides webRequestBlocking abilities except from extensions that are force-installed, or installed through policy for an organization.

That said, Chrome removing Manifest V2 will not kill uBlock Origin on Chrome. uBlock Origin's Manifest V3 support page says as much:

Rest assured, uBlock will continue to function on the Manifest V3 platform!

In practice, though, that means Chrome users who don't switch to Firefox will probably need to make do with uBlock Lite (which, for example, cannot yet circumvent YouTube's new anti-adblocker popup). Meanwhile, Manifest V2 will join the list of reasons that uBlock Origin works best on Firefox. A comparison of uBlock Lite vs uBlock Origin

Anyway, that's the challenge (and gorhill's workaround so far). You can follow gorhill's progress at https://github.com/uBlockOrigin/uBlock-issues/issues/338

It won't allow the extension access to the requests. This is good for securityin general, but that's kinda important to what ad blockers do.

The replacement is that they can provide the browser with a list of rules to block. The problem is that there is a max length to that list that is something like 1/3 of the current block list.