Technically it's the client's fault for changing their environment and installing updates that were not on any vendor-approved list. They made their system incompatible with your software by installing those updates. Unless they have a support contract with you, you have absolutely no obligation to provide updates to the software. I assume you sold them a perpetual license for the software, which allows them to use that specific version for perpetuity...all it means is that they are not required to pay a yearly license fee to use the software. Equate this to Windows...when you buy a retail Windows license, it's perpetual per machine. Microsoft provides updates as a gesture, but they are under no obligation to provide updates forever.

You can suggest that they roll back to a supported operating system version (either Windows 7/8.1/10 LTSB). Or if you would like to make some extra cash, you can sell them a compatibility update if you feel it is worth your time. Just make sure you make no promises about supporting it (unless it's for a limited period, like 30, 60, 90, 180, 365 days....but make sure you charge for that too).