Are you referring to the Manifest V2 extensions supported by Brave? The original extension creators were made fully aware of those plans ahead of time and have been in contact with Brave since then, e.g.:

https://github.com/hackademix/noscript/issues/359 https://github.com/uBlockOrigin/uBlock-issues/discussions/29... https://github.com/brave/brave-browser/issues/41173#issuecom...

I'm not sure how you can interpret forking open source codebases as a "shady" behavior (it's one of the most important reasons to use open source in the first place), but in this case there is a high demand for said extensions and Brave has provided the only way to continue doing so on a Chromium rendering engine.

(I am one of the devs who worked on the spec for this feature)

No, I am not talking about continued manifest v2 support. What I referred to took place a few years earlier when extensions where first implemented in Brave.