What is canon these days? Pull up a chair and have a snack ready.
First decide what you actually mean by canon.
Contentious point 1 - Traveller is an rpg rules system for adventures in a sci-fi setting, the Third Imperium (and its history and future history) is a setting for the game.
Too many people conflate the two and say Traveller is the rules and setting - nope, you have to separate the two to make sense of 'canon'. Thus you have rules as written canon and you have setting canon.
Contentious point 2 - since GDW produced the first supplements that detailed the Third Imperium the setting has failed to use the rules as written to describe it. The Imperium setting has no jump torpedoes, trade lanes are replaced by x-boat links etc.
Rules canon is easy, it is what is written down. Is there errata for the broken stuff, is there a consensus on the vaguely worded stuff that requires clarification? Original CT is pretty errata free but some clarification would be helpful (dig through 77, 81, TTB and ST and you get a pretty complete rule system with options). Later editions of the game have pretty severe errata issues, from MegaTraveller that took three print runs to fix the typos to T5 where we are still waiting for a playable game.
Rules and setting do overlap, the rules broadly describe what is possible in the setting (not always), sometimes the setting changes because of rules changes and sometimes the rules change due to the setting - sort of.
In CT the revisions between 77 and 81 edition made the rules conform closer to the setting, in TTB the setting was actually part of the book. Rulebooks LBB:6 onwards were written for the setting. MegaTraveller tied the rules and setting completely together but due to design system requirements certain setting details changed, jump fuel being the main issue.
Then we got a 'waking up in the shower' moment. In the past there had been changes to rules that changed the setting and changes in the rules due to the setting, but TNE changed the setting and the rules.
TNE was a deliberate attempt by GDW to undo the damage that MegaTraveller had done to Traveller (Frank Chadwick and Dave Nilsen interviews confirm this point), they did it by blowing up the setting and moving forward to an era of discovery, challenge. conflict, rebuilding and hope, while also changing the setting technology once again with HEPlaR.
A stab was made to move the timeline backwards to reset everything once more with T4 and forward in time with the T20 TNNE 1248 stuff. Along the way we had the GT ATU which was written by authors who had a particular vision for how the Third Imperium setting should be.
Mongoose decided to go back to 1105 and reimage the classic era, with new rules came changes to the setting, and now MgT has been revised for a second edition the setting changes once again with HG2e retconning the letter drives out of existence - again. And all the while MWM and his inner circle tinker with T5...
Contentious point 3 - every edition of the rules has changed the setting, sometimes in subtle ways sometimes in paradigm shifting ways.
Canon is for authors - write a generic supplement/adventure and all you need are the rules. Write for the Third Imperium 1105 setting then do a bit of homework and then write it anyway, it'll give us something to discuss
I look forward to a Mongoose Traveller Third Imperium sourcebook that solves these issues for the MgT ATU, but they can never be solved for the extant OTU corpus.