I just confirmed. Those running EOL Windows 10 can still manually install the latest updates from the update catalog and be fully up to date with cumulative and security updates. That might not include defender lists. It's a pita, but you can start with your latest installed update, enter it into the update catalog. Click on the one that matches your install. It will take you to a page with a link that says "superseded by kb50xxxx". Install that update and proceed up the chain until you're at the latest update. (punch that kb50xxx into the search bar and hit download) I usually recommend installing each update one at a time, not skipping to the latest cumulative or security update because sometimes they stuff in a servicing stack update which requires you to update in order. Otherwise if you know someone running an up-to-date version of Windows 10, ask them to list you off the KB numbers of the latest updates and install those working from oldest date to newest.