Well, windows update still works fine for all windows 10 versions starting with 1507. End-Of-Support does not equal End-Of-Life. Atm, 20H2 is pushed aggressively, but if it were to stop feature upgrades by some method, then the latest LCU for the edition (Home, Pro / Edu, Ent) will install just fine and stop nagging. Defender updates just fine even if the OS itself is outdated, but it's a sane idea to integrate it into the media like @nima1024 advised. I would go with a 3rd-party AV tho for any of these old windows 10 versions (pre 1903) where Defender lacks Tamper Protection.