Adblockers will still be allowed, they will just be crippled a lot. It will probably be the same as the adblocking situation on Safari.

If any 3rd party browser vendor wants to maintain a Chromium fork with Manifest V2, they can do so, but with the risk of code maintenance hell. They would also need an extension store for Manifest V2 extensions. Otherwise V2 extensions needs to be installed manually.

Browser vendors can also create their own separate ad blockers that aren't affected by the changes. For example Brave Shields, Vivaldi adblock, Opera adblock, etc.