Yes! The adjective "supersubtle" was coined circa 1590-1600 CE.
No, "subtle" does not have a prefix. It comes from the Latin word "subtilis" which means finely woven.
Her perfume was so subtle that he could only catch a hint of it when she passed by.
The prefix "co-" could be used with the act.
The prefix for "cavated" could be "de-" as in "decavated."
You could add the prefix af- to it to make the word affluent.
The prefix "inter-" could be used with "cept" to form the word "intercept."
The opposite of subtle could be obvious. Depending on the context, it could also be overt, harsh, forthright, or coarse.
Her perfume was so subtle that he could only catch a hint of it when she passed by.
The subtle differences in the shading of the bushes made it very difficult to spot the cheetah, but I knew that I could never outrun him.
"Possibility" doesn't have a prefix. You could add the prefix im- to it to make the word impossibility.
Renumber is a prefix numbered is a suffix
The current prefix is ex-. You could add the prefix in- to it to make the word inexcusable.
The current prefix is im-. You could add the prefix un- to it to make the word unimportant.
"Social" doesn't have a prefix. You could add the prefix un- to it to make the word unsocial.
"Cost" doesn't have a prefix. You could add the prefix ac- to it to make the word "accost."
"Patients" doesn't have a prefix. You could add the prefix in- to it to get the word inpatients.
Pre-The prefix for prefix is pre; because it's before the root word fix.
You could add the prefix ad- to it to make the word advantage.