Just 642 battery cycles but saying "Service Recommended"/

Make an appointment with Apple or an Apple Authorized Service Provider. Your cycle count depends a lot on use. My 2017 MacBook Pro cycle count is currently 79 so 642 seems unusually high. But, we use our devices differently and have different models. Sometimes comparing Apples to Apples isn't really comparing apples to apples. It's best to have someone look at it.

Cycle count has no relevance to battery health. If you never unplugged the charger, and the cycle count was 1, the battery would still degrade and eventually need replacement.

When remaining capacity drops to 80% Apple advices replace the battery. Of course if it fails earlier, it fails earlier.