Long-time Imgur user here, to provide (some?) clarity. The rules have been in place for years, and enforcement just as long.
There are two changes (of consequence) that are happening now:
1: The ban on nudity and sexually explicit content is being expanded from just public posts (where it has been around for years) to include private posts as well (i.e.: those that don't appear anywhere in the Imgur navigation system, but which you need to know the URL to get to). Sexually explicit content has been allowed here, but now is not. The stated reason is that "people were getting confused" because they could see a sexually explicit private post, repost it publicly, and then get banned, and were confused about this. Whether this is actually true or not? Who knows. I'm sure there's some trivial number of people who experienced this, but it's unconvincing as a major driver of policy change. Which brings us to...
2: They have announced they will now be using AI and machine algorithms to detect and automatically ban posts with nudity and sexually explicit content. IMHO this is the big one, and seems to be slipping under most peoples' radar. I personally kinda lean toward thinking the entire policy update is just wrapping paper around getting this bit in. Until now, all enforcement has been at the hands of human moderators; prohibited content can go up, and can stay up, sometimes for quite a while (half a day is not unheard of) until a moderator happens to notice, and it gets sent to the bit-bucket. These posts will sometimes garner dozens, or hundreds, of comments in the associated discussion threads, until it all disappears many hours later. (Including no shortage of comments mostly stating "Witnessed!" which is Imgur-ese shorthand for "I saw this post before the moderators deleted it! Ha ha!") My expectation is that (Imgur believes that) using automated tools will enable them to catch posts as they are made, and before they ever make it to the public pages to be seen by others. I think that #2 may be partially driving #1 for technical reasons (public vs. private is just a boolean flag that can be flipped on or off at will at any time - so if you want to catch posts at the time they are created, then they may still be technically "private" posts at that point, that may later be made public). I mean: yeah, there are probably ways of implementing #2 without requiring #1, but from a programming perspective, this is the easy/lazy way.
So yeah, I think this whole thing is just being brought about by Imgur seeing the possibilities that machine learning / image recognition offers, and wanting to give it a try.