Here are a few quotes that may or may not be faithful to their authors, in which the subject is a noun clause ending in a verb followed by a comma:
"What can be shown, cannot be said."
"What gets measured, gets managed."
"That knowledge which stops at what it does not know, is the highest knowledge."
"To tell a woman what she may not do is to tell her what she can."
Using a comma seems like a courtesy, for it helps the reader parse the sentence. But it does not seem necessary.
What do the various style guides say about this? Have any linguists weighed in (eg, Pinker)?