A key thing about the API is that moderating gets harder when the apps that moderators use to streamline/facilitate their work suddenly stopped working. Those apps relied on the Reddit API. These were created by and for the moderator community out of necessity.

Moderators had asked Reddit for tools and when Reddit didn’t provide they built their own. Then Reddit switched off the API without offering replacement tools.

That’s likely the primary reason that Reddit’s mods left and its content took a nosedive.