My iPhone 6s Plus is not holding a charge. Am I in need of a new battery ?

How to know if your iPhone needs a new battery

Check your iPhone's battery health percentage

Go to Settings

Select "Battery"

Tap "Battery Health"

Look at the "Maximum Battery Capacity" section

If your battery capacity is below 80% it might be time to think about buying a replacement iPhone battery.

That is highly likely. Realize this model was released by Apple over seven years ago.

iPhone Battery Repair & Replacement - Apple Support