The deprecated mode is only equal gain at +0 or -6 pan law. In between it looks like this. It was deprecated for a reason.
The default mode is equal gain at +0dB if "linear scale above -3dB" is checked, and linear at -6dB by definition.
I suppose... what we could do is deprecate both the 3.x mode and the "linear scale above -3dB" checkbox, and instead have an equal power vs equal gain control. Equal gain at other than +0 or -6 dB is not currently available in REAPER.