Hi Apple team.
First, you are not addressing Apple here but other end users. Apple does not respond here. We don't process complaints om Apple's behalf. We can provide facts:
— An iPhone 6 Plus can run no higher iOS version than iOS 12. YouTube now requires iOS 15 or higher. It was YouTube's (basically Google's) decision to set that OS restriction, not Apple's.
— I understand that WhatsApp has never released a iPad version. Again, your complaint is with WhatsApp, not Apple.
Since Day One, it has been the software makers' responsibility to support hardware abd OS version advances, not the other way around.
So... how about some links that actually let you reach those apps' developers:
YouTube Support
Whatapp Support