> They have instead hand picked exactly 4 manifest v2 extensions (AdGuard, NoScript, uBlock Origin, and uMatrix) and have hard-coded special support for them. They quite literally say in https://brave.com/blog/brave-shields-manifest-v3/

You're misreading that page, they have special cased the hosting of those 4 extensions, because they do not have their own addon web store and are relying on Chrome's instead. You can still install any manifest v2 addon manually, not that there are going to be many outside of those 4 that care about v2.

As for Opera:

"Today, we reiterate what we said back in October 2024: MV2 extensions are still available to use on Opera, and we are actively working to keep it that way for as long as it’s technically reasonable."

> for as long as it’s technically reasonable

Read: for as long as Chromium allows this via a flag.