If it's a new feature (and not enabling what we already have on Linux/macOS) then it's better if it remains inside an extension. I really don't know much about Kafka 😞
The extensions framework is not in 4.0 yet, but it is in 3.3.2. Nothing has changed there. The extension protocol is still compatible, so it's not a problem to build them with 3.3.2 and have them run within 4.0.0 (we will port the SDK soon!)
New features, provided they do not add dependencies, can be implemented right now
I realize the current situation is not ideal, but a lot has happened in the past month (a huge refactor from Facebook + the project has moved under the Linux Foundation)
Both moves have been really good for the health of the project! Things will return to normal once the first stable is out, at the end of this month 🙂