Shotgun slugs (the white shells) and buckshot (red) were both removed for what I've been told was "balance problems". Now, unless you go to science to ask for custom-made shells you have to craft, you're stuck with Incendiary Slugs for lethal ammunition.
I'm willing to agree that buckshot might have been overly effective against melee monsters, esp. terror spiders. Aside from a few cases, buckshot isn't much better than laser guns because of the pump/reload time and limited ammo. If it was removed for being too powerful, then NERF IT.
I can definitely agree that shotgun slugs were too powerful. There was a very high chance that when they hit an unarmored target it would cause IB and/or broken bones. The damage it dealt was manageable. I remember taking a single slug and teleporting away as a vampire only to slowly die from the IB, because no doctors or cargo techs would do surgery on me.
Currently the only easily accessible shotgun shells are incendiary slugs, which are really only great for people who enjoy themselves and everybody else in the room with them being on fire.
I've noticed a lot of stuff being removed from the game and not much being added. I think it's crazy that buckshot shells and slugs were completely removed rather then getting some sort of nerf. Leaving in incendiary slugs, which are a specialized form of regular slugs with less damage, doesn't make sense to me.