The Samyang lenses weren't DSLR ones with built-in adapters. They were bona fide mirrorless designs, the same that Samyang released for Sony E-mount cameras. I used to have the Samyang 14mm AF for RF mount, and it did not have a space in the back to account for the flange difference (and in fact was quite different optically from the EF 14mm AF lens which Samyang then did and still does sell).The Samyang versions were basically DSLR lenses with built in adapters. You could see the space for it when you looked at the DSLR and RF versions side by side. I don’t recall Canon ever going after Samyang. If memory serves, it was Viltrox (or another brand?) that got the cease and desist and Samyang proactively pulled their lenses in response to that. Perhaps to avoid what they thought might be future legal fees?
However, there have been some MF lenses for the RF mount that are more or less EF lenses with adapters. And a number of Sigma lenses for Sony cameras were DSLR designs, in the same vein...
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