The combined lens (50mm f/2 and a 0.71x speedbooster/telecompressor) will act like a 35mm/f1.4 lens in all regards (Same applies to teleconverters, in the other direction. lens designs exist that consist of a teleconverter or telecompressor behind a main lens, eg many 100mm macro lenses are 50mm macro lenses with a teleconverter in the back...).

DOF will be in the end dependent on aperture and magnification (subject to sensor). That truth is often arrogantly misinterpreted to say focal length doesn't matter re: background blur. It does, read on.

At THE SAME FRAMING, the actual DOF (in metres of depth) of the in-focus subject will, counterintuitively, be a bit shallower since you gained an f-stop of aperture.

A smaller acceptable circle of confusion for a smaller sensor will counteract this lightly, but in practice not by a whole stop.

The beginning and end of the DOF area around the focal plane might shift due to a change of camera to subject distance, which can also introduce apparent changes of DOF depending on what areas behind or before the focal plane matter in the picture.

Apparent background blur will still be less, since the change in distance needed for the same framing will decrease the magnification of the background.