Well, even if the Windows 10 is no longer supported, Windows defender should still be supplied with updates. I am running Windows 10 1803 on this computer and WD is still getting updates, even my Windows 7 partition that runs Defender is getting updates, although both do not get monthly security updates anymore. I am aware, it is a little risky but so far, I have not experienced any issues (My computer cannot upgrade to a more recent build due to unsupported video card)
In regard of third-party AV-Software, I am very hesitant because most of them have become too bulky that they slow my computer down to a crawl which Windows Defender does not