What could happen if one stops Windows 10 automatic updates and keep on using the OS? I have read that MS would stop providing security updates if one does that. Could MS also deactivate the OS?
Stop automatic update (if you can in Win10) only keep those security/hot fix/new feature/new driver/...., from been downloaded/installed automatically without your permission. It has nothing to do with OS activation. You can keep on using the OS without any update if you prefer. Those new patches to block illegal activation is another topic.