iPhone 6 iOS 15. My most used apps now want 16 or 17

I assume that you have the iPhone 6s mentioned in your tag line, for which iOS 15 is the end of the line. For the iPhone 6 mentioned in the title of your thread, iOS 12 is the end of the line.

Either way, if you want to run apps that require a minimum of iOS 16 or iOS 17, you will need a newer phone.

If you need to run a higher version of the operating system, you will need a newer phone.

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.3

• If you have an iPhone 8 or X, you can update to 16.7.10

• If you have an iPhone SE (2nd generation or later), or XR through 16, you can update to 18.3

Not an Apple issue. The app developers are not interested in supporting older phones. You need a new phone. Get one that supports iOS 18

it's the choice of app developers which version they wish to support

try

How to contact an app developer - Apple Support