Seems to be the case of now https://stackoverflow.com/help/licensing .

One of the more recent features added to SO was a timeline of activity and within the timeline you can see which license each the question is under. Here's an example for a random question https://stackoverflow.com/posts/49083680/timeline .

It's kind of weird that while the questions have a license declared the answer don't ... not sure what that's all about.

They're already periodically sending a snapshot to The Internet Archive: https://archive.org/details/stackoverflow.com

They can’t legally retroactively change the license on existing content though - and as soon as they change their EULA to say “You irrevocably assign copyright on your postings to us” then they’ll lose their thought-leader users - and the rest tumbles.