Do they still maintain it as in, it gets bug fixes and security patches? Technically yes, bug fixes and security patches are still produced for Windows Server. I wouldn't call it a good level of effort but it exists. They are happy to take your money for such a mediocre level of effort though.

Is it Microsoft happy path? No. Microsoft STRONGLY recommends that if you are running .Net (Core), you run under Kestrel on Linux in a container. Performance under Linux is much better than Windows.