I never personally used it in an interview (that's generally not how I do interviewing) but for someone experienced having not heard of recursion (and it's not

I never personally used it in an interview (that's generally not how I do interviewing) but for someone experienced having not heard of recursion (and it's not such a complicated subject, you wouldn't need much "experience using recursion" to be able to understand it) would mean they haven't really dug into the art & craft of programming. Just like I'd expect a moderately senior developer to be able to program in procedural, object oriented & functional styles even if they strongly prefer one to the others (and how would they know if they haven't tried all of them?).