My question is triggered by this question, which has some close votes due to being off-topic, and a comment saying that it is unsupported.

14.04 appears to be the most recently supported distribution with PHP5 available in the repositories. A question about 14.04 would clearly be off-topic at this stage.

But the linked question asks about installing old software on a still supported release (18.04). How should we handle this? At some point, the answer will probably be not possible, due to changes in libraries and so forth. One quick example is QT4-applications.

So when do we stop accepting such questions? Should we stop accepting them? Should we close them, or should we answer not trivially achievable when it's a huge work?