A system information app I originally wrote for Windows 98, but subsequently added support for NT-class Windows client, still runs and works almost perfectly today (with graceful feature degredation) on Windows 10. I do a tiny amount of work for every new Windows that's released, to support it. Sometimes no work at all. The codebase was originally written in 1998.
Nobody uses it now apart from me, but I still maintain it.
Nobody uses it now apart from me, but I still maintain it.