May I ask why Apple don't support IMG_texture_compression_pvrtc2 (https://www.khronos.org/registry/gles/extensions/IMG/IMG_texture_compression_pvrtc2.txt) when most of their hardware is capable of this PVRTC2 compression which has much better quality, sharper edges and can use NPOT textures?
I don't know the answer, but for what it's worth the iPhone 5s and newer supports ETC2 (if you use OpenGL ES 3 or Metal), and the iPhone 6 and newer supports ASTC which is a very good and flexible format (again if you use OpenGL ES 3 or Metal).
I guess they don't support it because it's a propriety format. ASTC is not propriety. (BTW. Apple recommend not using ETC2 without saying why)