Have an old phone no longer supported by Android? These OS alternatives can give otherwise working phones a second lease at life. — Bloomberg
Your phone's hardware is still in good shape, but the manufacturer has stopped supporting the software. In cases like these, you should consider installing an alternative Android version so that you can continue making the best of a perfectly good phone.
Smartphone manufacturers are notorious for ending Android updates after two to three years, despite phones being capable of longer lifespans.