The dynamic English language gains and loses words all the time. That isn't surprising.
But what about the words that only partly disappear?
It is common to change a positive word into a negative by using a prefix.
Common becomes the negative Un-common for example.

Common and uncommon are a positive/negative pair that are in regular use.
Some negatives have remained without their positive form.
Will you help compile a complete/incomplete list of these word orphans?
Here is my list so far:
Disgruntled - Gruntled?
I am not happy that there is no positive sense available here.
Disparage - Parage?
Should we only express a negative opinion of someone or something?
Incongruous - Congruous?
Is "congruous" incompatible with its negative form?
Congruous is actually a good word, "corresponding in character or kind", but it doesn't quite match the negative.
Indiscriminate - Discriminate (Discriminating?)
The positive here is actually used, but has a very negative meaning itself and isn't an adjective. It is used as a verb. Hence "we should not discriminate against" a group of people. That may be why it isn't used as a positive for indiscriminate.
Inviolate - Violate (Violated?)
Maybe this should be left for you to look up in a dictionary.
Nondescript - Descript? (Distinct?)
I have the feeling that I need more details about this one.
Unmentionables - Mentionables?
This one strikes me funny. Of course, there cannot be a positive here. Underwear is not to be mentioned in polite company (guess that leaves me out of polite company)
Happy reading.
--Algot