There are cases where old, barely supported or possibly unsupported server-class systems (be it software or hardware) are still being used professionally; for instance:
Low-budget operations (still commercial, just not big);
The system can still perform well for its intended purpose, and downtime is not catastrophic given the use so vendor support is not as critical;
Existing in-house-developed systems that would not be cost-effective to change make use of it.
What would be good guideline(s) on the threshold where such a system can still be considered on-topic here?