"Nonetheless" was once commonly written as "none the less." However, that that is no longer the case is indicated by the Online Etymology Dictionary, which – as @J.R. has noted in a comment – says that "none the less" was "contracted into one word from c. 1930": More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.