The OpenGL ES specification page about the GLSL function modf() indicates that it should be supported by version 3.0, and not by version 1.0.
What about version 2.0? Does it support it?
There is no version 2.0 of the Shading language accompanying any of the GLSL ES specifications. There is only 1.0x and 3.x. If your question then is, "is modf supported in versions lower than 3.0", it is not supported.