From Dictionary.com
Currently, inalienable is the more common spelling, but historically, that wasn’t always the case.
Most are familiar with unalienable after being introduced to it in history class while learning this specific passage from the Declaration of Independence: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.”
The dominance of unalienable versus inalienable has changed over time as preference has evolved over the years.
Unalienable was the most popular choice until the 1830s when inalienable began to replace it. Now, inalienable is much more common with unalienable pretty much reserved for that famous proclamation.
I never heard anyone use "unalienable"