Is the iOS still supporting iPhone 8?

Until Apple tell is it can't.

iPhone 8 supports iOS 15. The earliest iPhone capable of supporting iOS 15 is the 6S, a couple of years older than the 8 so you have some time to go yet. Regardless of that, even after it can't support a 'current' iOS it remains a very capable phone.

Nobody here would know an exact date. But Apple typically supports devices for 7 years after the last sale date. And, sometimes provides additional software updates for security or other reasons after 7 years.