There are no plans to change LÖVE from its y-down (with y=0 at the top) by default coordinate system. It's working as intended. 0.9's shader coordinate system was always y-down by default (try outputting the texture coordinate values and see where 0,0 is and where 1,1 is!), it's just the pixel coordinate value that was buggy and didn't match anything else.
I don't think I know of any pixel shader code that actually depends on the coordinate system of the pixel coordinate value in shaders being 0 at the bottom – the most important thing is that it should be consistent with the coordinate system of the texture coordinate values.
It is in 0.10.0 (and it is in non-LÖVE programs, even ones with a y-up coordinate system), but it wasn't in previous LÖVE versions so you had to do some manual fiddling to get non-LÖVE shaders working right if they used that. Now you don't (as long as you use the pixel coordinate argument to the effect() function, and not gl_FragCoord – which you should always avoid).