But about twenty years out is long enough that society ought to have changed, but soon enough that many readers will be around to see them, or will live in that projected "future" year with relatively easy access to the book to compare to reality. And that's tricky.
In this particular book, published around 1980, there were two timelines, set in the early 1960s and the late 1990s. And... well, the author made a few tiny gestures toward saying things had changed, but for the characters we actually spent a lot of time with, it was sometimes hard to tell the timelines apart. Gender relations and characters' attitudes towards them seemed about the same. I think there was a throwaway mention of it being more acceptable for young people to live together outside wedlock and perhaps even be gay, but our viewpoint characters in both timelines were predominantly housewives (which was the unquestioned default job for married women in this fictional 1990s) who talked about how science and business was over their heads, male scientists who were occasionally startled to realize their wives and girlfriends were independent people with opinions and feelings rather than props there to support the menfolk, and sexual predators. I'm less sure how the depiction of rivalries between elite British schools compares to the real 1990s, but it sounded an awful lot like the way things were described in books written decades earlier, so possibly it isn't just gender relations, but about 90% of all social relations which froze around (this author's version of) 1962.
Now, given the vast range of possibilities, I wouldn't expect the author to predict accurately what society in Anglophone countries would be like nearly 20 years out. But I would expect them to look at the dramatic changes during the decades prior to 1980, guess that the 1990s would look noticeably different somehow, and make more of an attempt to construct something that seemed plausible, even if it turned out to be technically "wrong" as a prediction. A sufficiently different-looking "wrong prediction" 1990s probably would have seemed more "right" than the "just like (the parts of society the author is familiar with or thinks are most important) today" version we got. This book even had the advantage of a multiverse with different possible timelines coexisting, so it could have been plausible as some version of the 1990s, even if it wasn't our 1990s. But as it was, the book felt painfully dated. Like, Jane Austen seems fresher and more relevant despite being far older. (Though maybe Austen just wrote more convincing female characters...)
For a comparison, Octavia Butler published The Parable of the Sower and its sequel The Parable of the Talents in 1993 and 1998, and set the story mostly in the then-near-future of the mid-2020s. Now, we aren't quite at the point of the United States being divided up into a few struggling walled neighborhoods which survive by eating acorns, a handful of corporate towns, and many disintegrating cities full of pyromaniac drug addicts... but these books still feel more "real" to me than the other one. Granted, I think Butler put a lot more thought into how society might change, since that's kind of the point of the duology, whereas Timescape was more about sending messages back through time if only the university will give you the funding to do the experiments. Okay, and also about how humanity will fail to get its act together to address environmental devastation until it's killing lots of people in rich countries, by which point it might be too late. Good call on that one.
But Butler had so many plausible possibilities, big and small, all woven together. (And female characters who don't spend 90% of their time thinking about either cooking for or having sex with the male characters.) Corporations increasingly smashing any gains unions had made and controlling some people's lives entirely? All sorts of additional creative ways for the powerful to revive slavery, either illicitly or with a legal fig leaf? Warming climate, dwindling resources, and an increasing gap between the rich and everyone else? Massive social unrest, including some spearheaded by an increasingly powerful fundamentalist evangelical Christian power bloc led by a president who promises to "Make America Great Again"? (No, I'm not joking.) The details may not match the way history has played out, but the general trends are pretty spot-on.
And more importantly, they feel like a version of history which could have happened, even if it didn't. (At least, not quite as quickly as in the books. We'll see how the decade goes...) Our MAGA president wasn't a handsome young preacher, but someone like that could have played a similar role as the one we got. Someone like that could still be in our near future, for that matter.
So, writing twenty years out is hard. But putting enough work to develop a plausible development of society during that time in can make the book feel believable and realistic even when we know it's "wrong," while not putting in quite enough work can make the book feel incredibly wrong no matter how many tachyons you throw in.
And seeing it done well makes it much harder to wade through one that isn't half as thoughtfully developed, even if that one has other good points.