FWIW, WhatsApp claims support back to Android 5.0, and if they haven't changed their support decisions since I left, that means there's a significant amount of users in the wild on Android 5.0. I'm not surprised Google only goes back to Android 6, they were always dropping versions from support before WA did; their threshold must be higher.
WhatsApp majority userbase sits outside of the US, and it's deeply embedded in many 3rd world countries. I'm not sure how their European userbase compares with the African/Asian userbase.
I reckon the WhatsApp userbase OS distribution skews much more to older android versions compared to an app that mostly enjoys US/1st world country userbase.
Certainly, the WhatsApp userbase is skewed. But most of WhatsApp Android users are also Google Play Android users (certainly not all, WhatsApp publishes the apk directly), so if WhatsApp doesn't want to cut off X million users on Android 5, but Google is ok with not supporting it, Google almost certainly has a higher threshold value required to keep support. I'd guess the Android 5 users with Google Play and not WhatsApp outnumbers the Android 5 users with WhatsApp and not Google Play, but none of those statistics are public.
With the number of Android devices out there, it really makes more sense to think about it in actual user/device count rather than percentages.
