Why do people keep assuming levels got "removed" and not that, in between real life and all my other work on this game, I did not have time to take every single old level imaginable, fix their perceived problems, bring their visuals up to par and then add VIS - because even maps like Hostile Waters that only have 3 small areas containing props/trees can suffer framerate issues simply because buildings are the biggest framerate killer? Bear in mind that I am having to do almost all of this myself.
Raap pretty much hit the nail on the head, Volcano is one of the hardest maps to make look good (an issue to which I just said "fuck it" for Metro since so many people wanted it to return but we also lacked any assets that could possibly make it look good - now A Path Beyond on the other hand, I actually want to see that map done justice) so since the only other mapper was doing big, glamorous projects that he didn't have much free time for, I put my time into other, easier projects so that we could actually have a decent number of maps, 16, in the initial release, and actually deliver said initial release before interest in the game died out.
The only levels that legit got "removed" are Hourglass, DM_Killfields, CTT_Luna, and Fissure. Hourglass was the only AOW map in all of Gamma that I actually hated (as opposed to just hating Gamma's unit balance/game mechanics) and I am definitely not alone on that, its layout makes it completely unsalvageable. Killfields is your typical no-barracks DM dreg with no strings attached, which almost everyone seems to want to skip, so why bother reviving it? Luna is in a similar boat minus the "no strings attached", but those strings would not have played well in Delta due to inf combat being redone to discourage jumping, and it's kind of obsolete now with Lunar Paradox anyway. And Fissure, again, suffers from a layout that simply does not gel well with this engine's infantry combat mechanics. When I have the time, or maybe more mappers step up and stop expecting me to revive another 15 or so old maps all by myself in a reasonable time frame? Sure you'll see other old maps return.
What about Pacific Threat then? Pacific Threat is basically an inverse Shallow Grave - water everywhere except a small island in the middle - and it doesn't seem to suffer this problem despite having longer naval base-to-base travel times than CI/Under. Fog, draw distance, and subs diving means the two lanes can't see each other, sure, but there are always other people scouting who report subs inc or boats inc wherever and this doesn't happen.