Are you sure you deleted them?

i don't have them in my design screen either and I'm certain I didn't delete them

i've never seen them in the design screen. |I could swear that they were never there in the first place.

afaik, pioneers are "hardcoded" defenseless settler units. They're the only unit that can perform that role, and they cannot be given equipment or traits, or altered visually.

You can delete your designed units, but not the models created by dafault. Those ones, you "hide" them instead. So if you accidentally hide one and want to recover (for example, a slave to get the "slave trait"), you have to check "show hidden units" button.

Pioneers and catapults are outside of this system. You will never be able to hide or delete them in the design screen, because they never appear there.