I didn't find any official doc for this.
Yes; it is part of the mandatory support required for OpenGL ES 3.0. Using mediump will usually have a performance advantage if the extra precision is not needed.
HTH.
Pete
But, Is that mandatory in opengles driver? I see a comment in r5p0 driver: "High precision. Only used if the target supports high precision". That means if GPU doesn't support it, it's meaningless if driver support.
But, Is that mandatory in opengles driver?
Yes; it is part of the mandatory support required for OpenGL ES 3.0.