In theory, the SR timeline diverges in '88 or '89. Though realistically, it should diverge even earlier than that to set up certain events that happen in the early 90's. If I were forced to write the "history" of Shadowrun, I'd probably have a few subtle differences around 1970 or so, and lead on up through into the '90s with the major ones that start defining the world of Shadowrun.
That said, SR's history has had a few retcons over the years, to incorporate events from the "real world". IIRC, someone slipped a Twin Towers, 9/11 reference into one of the early FanPro books, as an example. It doesn't really invalidate anything, since I don't think the Neo-Anarchists Guide to North America touched on the World Trade Center one way or another, so it doesn't invalidate earlier material... But at the same token, it was also slightly unnecessary, since we could easily explained away the lack of the WTC in Season 3 of Missions and the the Manhattan eBook by saying "The quake detsroyed them". Or they were torn down and rebuilt by Ares. Or whatever.
Personally, I think SHadowrun needs a hard break in '89. I'm not a fan of trying to force real world changes into Shadowrun. It's a different world. Things have progressed a LOT differently due to the Crashes, VITAS, the megacorp structure, the multitude of wars, civil and other wise... One of the things that always makes me want to bang my head against the table is anytime someone busts out "But it happened like X in the real world..." I mean, yes, obviously you want to look at real life as a template, but you don't need to be bound to that, and regardless you have to look at it through the lens of Shadowrun's world.
Smartphopnes, Wireless, miniaturized computers, etc... Sure, they exist today. Why did they take so long to materialize in Shadowrun (other than the real life explanation that they hadn't been invented yet, and the writers back in the 80s and 90's didn't think of it)? Simple... Corporate Greed. When 10 companies control 99% of the worlds economy, there's much less of a need for competition. And without serious competition, you don't have the constant advances you do now. Why make things smaller and more powerful, when you can keep them larger and less powerful, sell them at inflated prices, and your manufacturing costs are almost nothing? THat;s maximizing your profit margin right there.
Plus with things like VITAS and the wars that happened, it's easy to say that certain advances were simply never made. WHat happens when the first guy to engineer the smartphone dies of VITAS, or was killed by rampaging Amerinds, or was locked up in a concentration camp? What happens when Microsoft and Apple lose their entire database to the Crash virus (WHich also conveniently corrupted their entire backup system as well). And when half the programmers and developers who worked on those programs are dead due to various issues, who's going to be rebuilding all of this?
It doesn't all make sense, but it's there... We roll with it. It's a part of the fiction, and it's a part of the groundwork that the world of Shadowrun is built on.
Bull