My question is triggered by [this question](https://askubuntu.com/q/1157501/653515), which has some close votes due to being off-topic, and a comment saying that it is unsupported. 

[14.04](https://wiki.ubuntu.com/TrustyTahr/ReleaseNotes) 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?