Well, I think she wrote in the sense that Ennis seems to be shown as cut off from society a little. It's interesting that there is very little mention of the 'outside world' (current events), in contrast to another story in the collection where the characters' lives are tied in to what's going on around them. There are references to the draft, and the Thresher early on, and later we see that Ennis is affected by economic changes, but apart from that there's very little ... we're aware of the passage of time over 20 years, but barely aware of the massive outside changes going on between the '60s and the '80s. So Ennis seems to exist in his own world, and that gives much more power to his father's message.
By the prologue, he still seems to be a little cut off, and the book is vague about the date. It's maybe the '90s by that time. As I mentioned, I think the changes we see in Ennis are internal - his own gradual working through of what Jack meant to him. So it's just speculation about how things like AIDS and Matthew Shepard would have affected him - I take the point that he might not even have heard of these things, being more cut off than most people. I tend to think he would have heard of them, but it's interesting to imagine how he might have applied them to himself. Do you think Annie Proulx is sidestepping those issues altogether - almost as if she's setting Ennis in an alternate universe where the world continues not to change around him (as it apparently doesn't change during the 20 years - although we know it did, and others - Jack, Alma, probably noticed it - I think the film makes that point), or not change as far as he's concerned anyway? In that case, all the progress is internal?