I want to access all posts of a specific user using /api/---.
Already got that worked.
Not possible right now without a plugin, you can wait for https://github.com/NodeBB/NodeBB/issues/7217 to be implemented
I agree, that doesn't sound right.
We highly recommend you stay up to date for the latest security fixes.
This looks more like a cacheing issue though. You can try running a rebuild and restart, that should reset the cache buster.
It is a global privilege now.