I agree, some of the later books were more restrictive/didactic about what UPP codes meant and how much freedom there was to interpret them. I think some of that may have been because the OTU was developing and some of the writers were trying to ensure possible interpretations fitted into that specifically. Or at least that some of the other companies publishing Official material had a clear guide for how to develop material that would be consistent with GDW material.I think Classic Traveller is interesting because the rules - written in 1977! - hint at possible approaches to world-building that are more often associated with "modern" RPGs. (I think some of those hints were diluted in later versions, especially the GMing advice in The Traveller Book which leans hard into the standard railroading approaches that were emerging in the early-to-mid 80s.)
The main reason I like to have a full subsector to start with is so I can see potentially interesting worlds that I think the PCs might interact with a lot, and start working on some ideas for them before the PCs get there. There are certainly published adventures where there's both a GM and Player map, and the latter is far sparser on information than the former; Leviathan is the most obcious (and rather good too).In my current campaign I came to the first session with three worlds already rolled up (a hostile atmosphere domed-cities world; a water world (hyrdo 9 or A); and a low-TL world with a disease-tainted atmosphere (this was pre-pandemic, should it raise any questions about my good taste!)). I rolled up a starting world after the players had rolled their PCs, and together we helped make sense of it (which is something suggested in Book 3); and then rolled a patron encounter, and related that NPC to the PCs backstories that had been worked out in the course of rolling up PCs and world, and used my pre-prepared worlds to help frame her mission for the PCs.
Building things up as you go is one technique, whether it's adding new systems to the map or actually getting round to detailing them a little more. I admit, much of my own setting built up over nearly forty years of gradually adding more and more, either because players wanted to go somewhere new or because I wanted to run something that wouldn't work so well in the existing parts of my setting. Originally I started with a rough-and-tumble region outside the main empires (all I had for those was names) but when the players were interested in playing spoilt noble brats touring the frontiers of their realm and going into the outback beyond, well I had to develop an area on the fringe of the empire for that to really work.Over the course of about 20 sessions more worlds have been generated, and I've had to locate them all on a star map (though I don't use the canonical sector/subsector arrangement) to keep track of them. I think this is inevitable (or at least hard to avoid) in any "leave blanks" game, especially one like Traveller that places a strong emphasis on the travel/explorative element of play - over time, those blanks get filled in.
Something I'm doing more is making "maps" in the Fate style, with zones rather than a typical Traveller hex map. So a world with one major and one minor continent and 70% Hydro would have a Panthalassa region (isolated islands, aquaculture ships, musterious creatures), Pangaea (huge desert interior, huge mountains, coastal settlements) and sub-Polar continent (exiled criminals, animal herding, raiders). It's an easy way to get a rough idea of terrain types and add more details later rather than work them out at the start.I've never used any world maps. After an unhappy initial experience with the rules for onworld exploration, which are the one area I've found Classic Traveller to underpeform, onworld exploration hasn't been a bit part of our game. The PCs mostly travel from A to B on a world via their starship or ship's boat, and as far as ATV travel is concerned I've used encounter and evasion rules to manage the pacing rather than focusing on how many miles the PCs have travelled in what precise direction.