Ungiven is listed in Merriam-Webster, the Shorter Oxford and Wiktionary.
The two examples you give, ridethetalk, show the word meaning not partial to. It can also have the more literal meaning of not given. Here in Victoria, Australia, the Transport Accident commission used "Ungiven Gifts" as the theme of a
pre-Christmas road safety campaign in 2013. And Gifts Ungiven is apparently the name of a card in the game Magic The Gathering.
The word seems to crop up also from time to time in sports reports, in relation to umpiring decisions, or non-decisions:
Error-wise, the ungiven penalty and the unshown card are no more or less controversial than the given penalty and the shown one.
I'll add
ungiven as a rare word.