Anyone still using Windows 10 mobile will probably be much safer using it without any further security patches as I doubt that there will be many hackers, virus coders, malware writers even considering going after such as small install base. After all there are many more unpatched Android handsets in everyday use due to the manufacturers either being slow to patch against vulnerabilities or just don't release updates at all.
I used my old phone for over 12 months without any security patches from the manufacturer.