It's pretty formal. You certainly don't here it in everyday speech here in the US. From NBC Dateline: "There was the Leppink murder practically begging for a solution, so they re-interviewed the cast of characters now scattered nationwide. And what do you know? A remarkable piece of evidence, hitherto entirely unknown, jumped right up and said hello." "Hitherto unknown" is a common collocation. Google-search it. You'll see a lot of examples of that. I've never used the word in my life, and I'm too old to start now. ; )

not super common