The problem is a mountain of older ipads and iphones and ipods that aren't eligible for the latest major ios upgrades, and thus get stuck with an outdated safari/wkwebview/webkit implementation. This is only going to get worse as the EOL'd ipads these days are quite powerful and usable otherwise. And due to Apple policies banning bring-your-own-html-engine, firefox or chrome can't save them either :(
Most of my Safari compat issues are on older iOS devices that don't get Safari upgrades, for better or worse those devices do last a long time and a lot of people resist upgrading Mac software.