I am a fan of Traveller even though I only refereed the game only a handful of times. I bought products across many eras of Traveller. I bought products in just about every edition ever produced. James of Grognardia has a pair of posts on Traveller, here and here.
In these posts he talks about the Spinward Marches and in the later post has some good comments on "evolving" settings.
I agree with his sentiment and vastly prefer Harn's approach of an eternal present over a campaign world that "evolves". That my plan for the Majestic Wilderlands. I may post or write something about what happens post 4436 BCCC in my campaign but I won't make it official. The idea is that referee will be free to resolve things like the Viridian Civil War without me writing something that contradicts their resolution.
But the Spinward Marches is not a particularly good example of an evolving campaign. Yeah there was a 5th Frontiers, yes the Imperium and the Zhodani changed from their original presentation. There are versions set in 1065, 1105, 1117, 1202, and 1248 as well as the GURPS version and the Mongoose Version.
But after the first couple of years added details pretty much stopped. Later products rehashed the library data of older products. It was a bit of mess. Even with TNE smashing the Imperium the Spinward Marches was left pretty much alone as a concession to Traveller fans wanting run traditional adventures with the TNE rules. Plus the date 1202 was so far off from 1105 or 1117 that you would have to run a campaign for a really long time to the point where it even a factor.
It wasn't until the advent of GURPS Behind the Claw (not so good) and Mongoose Spinward Marches (better) that we finally get systematic detail to entire Spinward Marches. But now comes the danger that James points out. Will Mongoose resist the temptation of "advancing" the setting or choose to focus aka Harn on added detail to the existing line. We will see.
Finally there is Traveller 5 by Marc Miller. I am part of the beta group. I can't give out specific detail but one of the goals is to support "eras" of the Third Imperium. That the referee picks which era he likes and goes off and uses that. The sentiment seems to be "OK we got this rich background let's give the tools to flesh it out." as opposed "let's advance it even further".
It is interesting concept and it may very well work. That is if they ever get T5 out. Some remarked about my first Points of Lights that they like how each region were set in a different time of the quasi-setting I used. I may explore this in a specific line of products after I get a few more Majestic Wilderlands products out.