I'd argue that's less true than people think. NASA has not stopped coming up with risky plans they just do what any business or agency HAS to do and manage the risk.
The best argument I think you can come up with for this is STS-1, which was a pretty big gamble - putting crew on a space vehicle on its very first launch. And it was a vehicle with all kinds of new failure modes - and as we know, we very nearly DID lose the crew. There was no hiding the risk on that one.
But that *was* 37 years ago. The closest thing I have seen to that recently was NASA's initial plan to launch the EM-2 crew on what would have been the first flight of the SLS Block 1B with a previously untested Exploration Upper Stage (EUS) - sadly, because the damned thing is so expensive and production cadence so low that they can't easily afford another test flight. Then again, the Astronaut Office was still fighting that one, I hear, when they finally had to push Block 1B back a few years; and at least Orion has an LES. The Shuttle did not, unless you count those suicidal ejector seats on STS-1 through STS-4 (Bob Crippen: "Survival was not very probable.")
Generally speaking, though, I think NASA *is* a lot more risk averse, and it would take something truly extraordinary to throw that out the window.
Had an "13" accident happened to Apollo 8 they crew would have likely died no doubt but the odds were vanishingly smaller than "50/50". Yet most of those same "mangers" willingly went with odds a LOT worse for every flight of the Shuttle because they didn't see a 'choice' and firmly believed that "failure was not an option" which they got from Apollo.
I'm going to disagree on the latter assertion.
On the former, risk analysis is necessarily a difficult and even arbitrary exercise, and only as good as the data it is based on. Kraft may or may not have been exaggerating, but based on what they knew, NASA managers had reason to think the risks were high on Apollo 8. Of course, had they known about the SM oxygen tanks, lunar mascons, solar flares, micrometeorite risks...their systems may have been (mostly) better than they feared, but there were plenty of things they did *not* know.
You can say that Glenn Lunney and Arnold Aldrich (to take two examples) were old Apollo hands, but it's also not evident that they had accurate information on certain risks with the Shuttle. The real problem was that so many of the middle managers in the program were *not* Apollo veterans.
I'll have to read that one but if that's a quote from it then it shows the authors bias pretty baldly. (Gahhh, have to find a library copy that's NOT a cheap book,
https://www.amazon.com/Apollo-Race-Moon-Charles-Murray/dp/0671611011)
I'll just say that there's a lot of people who think that it's the best overall one volume history of Apollo that's been written. (This might be the only book I know on Amazon with more than 20 reviews that has 100% 5 star reviews.) It's true that Murray leans libertarian, but I don't think the problems he points out in post-Apollo NASA are without foundation. At any rate, it doesn't really come up in the book.
If you really want to skim it, by the way, you can find it online
here.
Coming back to it, NASA of Apollo COULD do a Venus flyby and would do so under the right circumstances and so could today's NASA. Those circumstances are the key of course.
Again, though, the question is not whether NASA was *capable* of putting together an Apollo-based Soyuz flyby. But the risk level was
extremely high even by Space Race era standards, even based on what NASA managers of the time knew, and I think something v
ery, very extraordinary is needed to get them to commit (and sell it to Congress). I think it's much more plausible to see them opting for an extension of the lunar program if they're looking for another brass ring - the key factor being confidence in their life support systems. Even a three week AES/ALSS mission to the Moon is not remotely as demanding in this regard as a 14 month interplanetary flight. NASA in 1972 had never put any human being in space for longer than two weeks. Even Skylab topped out at only 84 days. Today, of course, we have far more experience.
I do think it's easier to see the Soviets attempting it, given that they generally had a higher risk tolerance than NASA did, and it would make more sense to them as a catchup move given their capabilities. Of course, I'd give them no better than 50/50 odds of getting the crew back alive...
I will say that a Venus or Mars flyby mission in the early 70's is pretty hard to justify, even as a political stunt, in terms of return on risk. All of the Apollo missions were pretty high risk, but at least you were getting onto a surface, and able to do a surprising amount of scientific return even in a single EVA. There's literally nothing in a planetary flyby you could do in the early 70's that requires human beings; if anything, that is even more true today. And once you add in the solar flare danger, well...I don't see how any sane man could justify it.