Usually it's important tasks that are difficult and expensive to retrofit.
I had a customer who this day has a distributed system running Windows NT4 on Alpha for s critical business system. It was cheaper to setup a dedicated network then to deal with the application. IIRC, they now have a 5 year project to replace it that is just starting.
We have a few machines that are controlled by software that only runs on NT4. They've been air-gapped and run on their own domain for security reasons, but we can't get rid of them without spending millions on new machines (this is a high-tech manufacturing environment). The vendor of the current machines no longer exists and in those days we never thought to include software escrow in contract negotiations ... not that we'd have the gumption to attempt updating industrial machine controllers ourselves anyway, but still.
I had a customer who this day has a distributed system running Windows NT4 on Alpha for s critical business system. It was cheaper to setup a dedicated network then to deal with the application. IIRC, they now have a 5 year project to replace it that is just starting.