I had to look this one up, because most of the dictionary entries were, as others have noted, meaningless.
This is perhaps the best description, from Hosbeg, of all places:
Both insurmountable and unsurmountable mean the same and are interchangeable.
Of the two words, insurmountable is the preferred version. Unsurmountable was considered wrong some years ago, but today it is gradually becoming an accepted form of a difficult situation or problem that is too difficult to surmount or overcome.
Unfortunately, it's completely wrong! But this site, from kris-spisak.com provides the best description of when to use "un" vs. "in" (i.e. it's determined by whether the root word is Germanic or Latin).
Thus "unsurmountable" has NEVER been correct, no matter how often it may have been used. The correct form is always "insurmountable", since it's based on a Latin word.
P.S. This is what happens when dictionaries quit hiring qualified human editors, and instead turn to computer generated bots to generation both dictionaries and examples in their dictionary entries.