True stereo convolution means the reverb takes *two* impulse responses, as if two speakers have been placed in the room, and there's a stereo IR captured for each one separately. This is different from the currently available mono to stereo, where a single speaker is placed in the center of the room, and the resulting reverb is recorded in stereo - but it's a stereo reverb of a mono source signal. While there are options for processing inside Open AIR (cross-feed, cross-delay, assymetry) that can be used to fake how the differences between left and right input, it is impossible to use actual stereo to stereo IR files without resorting to tricks using the Splitter and two instances of the plugin. This gets unwieldy, especially when the need arises to try a different impulse or tweak some of the plugin settings - this requires changing it in both instances.
True stereo convolution is used by some plugins (Altiverb, AudioThing Fog Convolver, EastWest Spaces, Steinberg REVerence, LiquidSonics Reverberate) for increased realism. If introduced, it would allow to transfer the IRs from these to Open AIR, letting the user to stay with stock plugins for improved project file compatibility with others who might not have 3rd party plugins available.
This is nicely illustrated in LiquidSonics Reverberate user manual: