[msla]

Better question: Is there a fixed answer, or does it vary arbitrarily?

[mysterydip]

Good questions. I don't know for sure, but I know a couple of my apps have been delisted because I haven't updated them (they worked just fine and since then I upgraded and lost the development environment used).

I was unable to find anything specific beyond "apps we determine to be outdated" here https://developer.apple.com/support/app-store-improvements/