I'm not sure, where IS the shutter on an R or RP? Is it farther than 5mm from the sensor? Canon has patents for sensors that can take shutter-look photos without shutters. It's possible this lens simply wouldn't be compatible with RF cameras with shutters. Nikon and Canon both had lenses in the 70s that required the mirror to be locked up so they could get closer to the film, so there's precedent for certain lenses that don't work with all bodies.And where would the shutter go?
Someone pointed out that the Ra is aimed at 'deep sky' astro, not the wide angle star scapes that Canon lenses tend to suck at due to coma.They will release such lenses only if they see money. I have zero idea how large the astro market is but they did intro the Ra so there's that.