It's rigorously studied but it's a crap thing to put an arbitrary restriction on, because it doesn't correlate well enough to warrent that. If you put a restriction on it, bad code still gets through fine while some good code gets made worse to bypass the checks (the biggest one is large switch statements acting as dispatch: Perfectly understandable, but if you split such a thing into multiple functions magically the cyclomatic complexity goes down to satisfy the linter but the code has just gotten harder to understand for static analysis and humans alike).

A function can be five hundred lines long without having high cyclomatic complexity.

The average size of functions is independent of the programs total cyclomatic complexity.