Our current policy is to only permanently delete content that is illegal or otherwise too risky to keep. And there's always a chance of false positives, so it's easier to keep the art and remove the tag instead of hunting the art down again.
AI art was prohibited precisely because it was expected that users would flood the site with it and drown out legitimate art. We stem the tide by manually deleting intentional AI uploads, which penalizes the uploader by filling up five upload slots per manual deletion and prevents them from uploading more. If the uploader continues, they usually get banned to stop them from uploading entirely.
We do allow content that is AI-assisted, meaning there was significant artistic effort such as repainting or that the AI is limited to more minor parts of the image like background elements. But they're still considered borderline content and are less likely to be approved if there was a significant amount of AI contribution. The more work done by the artist, the better.