Specifically, if an iPhone can no longer run the latest version of Apple's iOS operating system, it will miss out on many critical security updates. Vulnerabilities that aren't fixed can put you at risk. Even though Apple makes its devices last a long time, once official support ends, the devices stop receiving software updates. This can lead to a multitude of errors, crashes, and even affect device performance.
So, if your iPhone belongs to the category of vintage or outdated, you'd better upgrade it to a newer model. You can continue to use your device if your iPhone is no longer receiving software updates. You're only using an older version of iOS and can't use the latest features. You'll continue to receive security updates, but not as often.
That way, you can keep using your iPhone safely. While it's just an estimate, Apple currently offers iPhone models for about seven years, so this should give you an idea of how long the replacement will last (assuming the hardware holds up). The only real option that allows you to use the latest version of iOS is to buy a new or used iPhone that continues to receive support. The Cupertino-based company has announced that it will discontinue support and service for older iPhone models, labeling them as old.