I have the same game, that was the first one I bought when it came out! But then, I also bought D&D when it first came out, along with WarpWar, Ogre, and Rivets!!! Holy Cow that makes me old. But at least now when I play Traveller I play characters closer to my own age, or younger.
I dunno about cover per se by the rubble alone, I think it's more a combination of a lot of factors. Stealth technology wasn't thought of much by the public back then (though HAVE Blue started in 77) so other than the chill cans in Mercenary you don't have much described officially by way of stealthiness and camouflage. I took my cues for that from Haldeman's
The Forever War, in addition to Heinlein. Haldeman described a more solid state cooling system for the suits that worked to help keep it IR masked, but still had vulnerabilities.
But also you have to wonder how much rubble even gets produced, and if it is, then how much background radiation, fires, ECM screening, and other stuff is going to "help" you use the rubble for cover? Does the military doctrine in your universe dictate that nuclear weapons and meson bombardment is limited only to most extreme cases, and built up areas are to be preserved as much as possible. So instead of rubble hiding you from tanks and meson artillery, you're back to man vs. man in an urban environment trying not to reduce the TL of the planet by more surgical, counter-insurgency combat?
You have to imagine that if the Imperium ever wanted to it could fry any planet at will without losing a single Marine to enemy fire on the ground, but you can't do that all the time or you'll just have open revolt even from the nobility in charge of the subsector you would be attacking in.
So instead it's like they say in
Starship Troopers (to paraphrase: "You can have all the biggest cannon, missiles, bombs, but in the end it takes the dogface with a bayonet to winkle the enemy out of his hole." Or something like that.
Here's some geek cred! The original BD: