It us still supported.
Am I the only person that read the line "In an effort to avoid a massive security breach, the IRS has agreed to pay Microsoft an undisclosed "premium fee" to continue to support and patch its servers".
They have an infrastructure that works, or at least I presume it does, so the additional costs for the continued patches and support will be an element in the whole company IT cost. Changing to a new OS may well mean different platform versions and so many of their systems would need changing and testing. If it ain't broke, don't change it.
Biting the hand that feeds IT
