Bumping up other entries is not viable. As stated, it wasn't earned.
The problem is that an author has that right to be forgotten so by keeping their stories around against their express wishes is legal hot water. The Ts and Cs about granting the site usage for 12 months probably won't fly if it was contested against GDPR, for example. Not sure. Don't know any lawyers.
Probably the best option, on deletion, is to reassign any comp title and author to a placeholder user account or stash the metadata in some placeholder table. Then, when rendering the comp lists, if a real entry is missing, look it up in the placeholder table so the name and title can be preserved, with no link to the actual story. Maybe a (story deleted by author) line as explanation.
That also makes it possible to manually add the few entries that have been deleted already to this database table to backfill the existing gaps. We know all winners as they're announced in a forum thread so it shouldn't be too onerous as a one-time exercise.
Please browse my digital bookshelf. In this collection, you can find 125 full stories, 10 micro-stories, and 3 poems with the following features:
* 30 Editor's Picks, 82 Recommended Reads.
* 16 competition podium places, 11 other times in the top ten.
* 23 collaborations.
* A whole heap of often filthy, tense, hot sex.
Sorry to be contrary, but I think there's a case to be made that the writer should have control of their own material. Stipulating that it has to stay on the site for up to a year is fine. Even five years, if it's stipulated up front. But author's rights should return to the author eventually. Publication rights to the story should not be held in perpetuity by Lush. The writer of the story should be able to do what they like with it.
I am unlikely to post any stories from Lush anywhere else, save maybe if I am going to self-publish them in a e-book or something. So I personally have no problem with the idea of giving a perpetual license to Lush for any comp winners I have (I haven't been in the top 3 yet so it is still hypothetical for me).
I agree it looks a bit awkward when a Comp Winner/Place leaves but I'm not sure I have a better idea than how it is handled now. Personally I would not be interested in moving from the Top 10 to a Top 3 just because someone chose to leave or delete a story.
A compromise might be links to the Top Ten instead of an empty space.