Dolby Pro-Logic decoding is no longer offered in A/V receivers (since 2014).
Although SQ decoders aren't designed to decode Dolby Surround, the Dolby Surround encoded surround channel is the same as SQ encoded CB (180 degrees out of phase in Lt and Rt).
I'm interested to know how well the Shadow Vector SQ decoder will decode Dolby Surround encoded content since there's quite a lot of content available (VHS Hi-Fi videotapes, LaserDisc videodiscs, some DVD & Blu-ray discs and TV shows from ~1990 onward).
Of course, the modified Dolby B NR used on the surround channel in Dolby Surround won't be decoded.
edit:
I see Dolby Pro-Logic 2 and QS decoding are included, so the only remaining issue is the modified Dolby B NR part of (original) Dolby Pro-Logic decoding system.
Kirk Bayne