Sudden battery drain on iPhone 6s

Batteries degrade over time with usage due to chemical aging.

You can expect about a 1% drop per month with average use on an iPhone. This is an approximation, that is why it may be plus or minus a few percent on a month to month basis.

Apple recommends replacing it when it reaches 80%. To check your battery health, go to Settings > Battery > Battery Health.

iPhone Battery Replacement.

iPhone Battery Replacement - Apple Support

About Lithium Ion Batteries

iPhone battery and performance - Apple Support (IN)

Tap Settings > Battery > Battery Health and check the number for Maximum Capacity.

If Maximum Capacity is 80% or less, it is time to replace the battery at the Apple Store or authorized Apple service shop.

The bottom line is your battery is old and failing and needs to be replaced. Your phone is 6 years old and if the original battery has never been replaced, it's well past it's usefull life.

Apple recommends replacing it at 80% but that number isn't etched in stone, batteries can fail before they reach the 80% mark.

Make an appointment at the Genius Bar to have the battery replaced.

Genius Bar Reservation and Apple Support Options - Apple

This is what lithium ion batteries do when they are near or at End of Life (EOL). Replace the battery.

Genius Bar Reservation and Apple Support Options - Apple

Your battery is running out of steam. You can make an appointment to have the battery checked and tested at the Apple Store, but it is clear that you are going to need a new battery soon, so you might as well go ahead and complete that task now.

If you don't want to replace the battery at this time, your battery is not going to improve one bit, and you should expect further degradation in performance.

It's your call.

I strongly suspect it’s the battery.

Are you the original owner? Has the battery ever been changed before?