I was using it to automatically scale a 4:3 anamorphic clip to the project size without messing with transform effects. Is there a similar effect to what the rescale did?

Oh, the rescale filter is normally automatically added to all clips added to MLT, so I thought it was useless... but if you really used it with some results I will revert the change..

Could it be an option on the clip properties instead? To format the clip (scale or stretch it) to the project size. It would work for this use case even in a simpler matter.

I can use the 'Aspect ratio' option in the clip properties with the same results. I think it's not a problem to remove rescale if it wasn't an effect that anyone used.