This seems to have been intentional.
Simply saying that 16px is “default”, after years of our forums being at 15px, isn’t (IMO) enough justification for this change, which was almost hidden in the release notes.
@awesomerobot Was this change in response to community requests? Feedback from accessibility experts? There’s no context or explanation in the pull request.
I’m of the opinion this should be rolled back. Let it be the default size for new installs, but it should be opt-in for old forums.