When I audit the PWA, I get the warning: Does not provide a valid apple-touch-icon

You may have configured the PWA by uploading a circular logo. If you have a square icon and upload it, the issue should be resolved. It's not an absolute necessity, but it does work better on iOS devices. For an ideal appearance on iOS, when users add a progressive web app to the home screen, it must point to a non-transparent 192px (or 180px) square PNG.