C++ is C with libraries to hide some stuff and add others. Removing C++ OSes from your list, I'd say that you're down to a vanishingly small percent of deployed systems. So yes, knowing C is important.

And I have never met a single person who knows any of your remaining languages who isn't at last passably familiar with C. I've never asked, but I don't think any of them would call their C knowledge unimportant or not useful.