If your phone's battery charge is going from 100% to 1% in ten minutes, that strongly suggests a hardware problem of some sort. Any claim along the lines that a drop like this is "Apple's way of slowing the older phones down to encourage people to buy the new version" is nonsense.
iPhones do throttle performance once their batteries age enough. The alternative is unplanned shutdowns that would result from the phone trying to use more electrical power than the battery could deliver. Apple originally did not tell people that they were doing throttling for this reason, but these days they are up-front about it.
iPhone Battery and Performance - Apple Support
The iPhone 6s came out in September 2015 – more than 100 months ago. Posts on these forums often say that it is normal for the health of an iPhone battery to decline by 1% per month, and that once the battery health drops below 80%, the battery is a good candidate for replacement.
If your iPhone 6s still has its original battery, and the estimated battery health is still 81% after 100+ months, that is a remarkably long service life for that battery. Maybe the 81% is too optimistic and it is time to replace the battery. It looks like Apple still offers battery replacement service for the iPhone 6s.
If the problem is not due to the battery, there might be (guess) a short-circuit somewhere in the phone that drains any new charge quickly. In that case, I don't know what sort of repair bill you would be looking at – or whether you might need to purchase a new phone.
Either way, it sounds like you need to take the phone to Apple or an Apple Authorized Service Provider to have them look at it, and give you an estimate of what needs to be done, and how much it will cost.
iPhone Repair and Service - Apple Support
