It’s bullshit that the opposite of “impeachable” is “unimpeachable” instead of “peachable”
These are called orphaned negatives and English has loads of them. A great article about them is here: https://stephenliddell.co.uk/2021/03/17/a-gruntled-look-at-orphan-negatives/
As a slight tangent, a similar peculiarity in English (which I don’t know of a name for) is where you can use the opposite words for similar actions, e.g. you can chop a tree down and then chop it up.
Great link! I love the little story in there.
I actually use “shevelled” alongside many other words which to my mind “should logically exist” - for example, at the weekend I dismantled and then remantled a wall in my garden.