British nuclear subs were running Windows XP until at least 2017. It's easy to google, but the best article about it is No, Trident doesn't run on windows XP (https://ukdefencejournal.org.uk/no-trident-doesnt-run-window...), which ironically makes it very clear that Trident subs were running on Windows XP and had no plans to replace it.

Most UK government excuse: "The programme undertaken by the Royal Navy and BAE Systems to equip the fleet with a Windows-based command system was completed in just 18 days."

Translated: "You couldn't do better in 18 days, so you don't have a right to worry or criticize. Also, don't ask why this was pushed off until the last 18 days of the project."