As I recall, that was only in his novelization of the first movie, something that he himself stated was not to be considered canonical.
No, it wasn't only there. That was just one example illustrating his philosophy. I already gave the second example, of asking audiences at conventions and such to pretend the Klingons had always looked that way.
As far as the Klingon thing, that may have been his intent (a visual retcon of sorts), but ENT would later establish that that there was no retcon and that Klingons really did look like that back then (wish we could see some TOS Klingons on DSC, but that's neither here nor there). Not sure that really "counts."
I'm not talking about the "facts" onscreen, I'm talking about interpretation, the philosophy you bring to a work of fiction. Creators like Roddenberry understand that what they create is unreal and mutable, because the act of creation is itself a lengthy process of trial and error and change. Creators of TV shows and movies in particular are aware that much of what ends up on screen falls far, far short of what they imagined due to various compromises and limitations, so they're often disappointed in the result and would gladly change it to something closer to their original vision given the chance.
Admittedly, I don't exactly see Roddenberry as "word of God" on the franchise, given that not only was a good chunk of it made without him, he also wasn't the only creator who made it.
It's not about "word of God" or anything like that. I'm just pointing out that fans are free to take a Doylist interpretation of Trek, to accept the inconsistent details as just quirks or errors of the artistic interpretation rather than being "real" in the putative reality being portrayed. Some fans think that the only way to be "true" to
Star Trek is to be obsessively literal about every last detail, that any divergence from that would be a betrayal of its creators. I'm pointing out that Roddenberry, like most creators, would've thought quite differently. He openly invited fans to be flexible in their interpretation of the franchise, to acknowledge that it was an imperfect creation and that they didn't have to take every last bit of it as immutable gospel. Of course we don't need Roddenberry's permission to use our own judgment in deciding what parts to accept, but it's significant that we have his permission. And it's likely that every subsequent creator has felt the same way, for the reasons I mentioned above.
I guess that's why I don't see the need to view the franchise as a TV show within a TV show; beyond the fact that the source material itself is presented as being reality within its own world (as if we were flies on the wall), it feels like it's trying to explain something that doesn't need explaining.
Don't take it so literally. The "show within the universe" conceit Roddenberry used in the TMP novelization was just that, a conceit. It was a figurative, playful way of telling the audience that he saw the movie as being a truer version of the reality he envisioned than the show had been, that he was taking the opportunity in the movie to improve on what he'd done before. It was just a cheeky way to handwave the changes, not something I'm suggesting you should take as a literal in-universe fact.
There's actually very little in common between the DSC entity known as Control and the one David Mack wrote about. Very little.
An aside: it's kind of annoying that Memory Beta doesn't make the distinction between the two clear, because their backstories are incompatible and it gives the wrong impression.
Except that Dave's Control/Uraei is known for putting up false fronts and making people think it's been defeated. What we're seeing on
Discovery could simply be another one of its misdirects, or perhaps an experimental offshoot of itself that went out of, err, control.