It’s a combination of minimum security requirements and features of the OS as well as the developer of the apps. WhatsApp stopped supporting older iPhones and my parents had to upgrade their iPhone5 which is over 13 years old now. I used to use my old iPad mini 4 only for Zoom until they stopped updating it and now my iPad is useless after using it for 9 years. Zoom blocks it at a server level so there is no way to bypass it.