[kupiakos]

I'm unsure what you mean - they said that there should be a space there and Merriam-Webster seems to support that. They put "shut down" and "shutdown" in the same page, and state that the verb version is "shut down". In the "First Known Use" section, it looks like it's just going with what the name of the whole page is.

[hombre_fatal]

They added the parenthetical "it should be 'shut down'" in an edit.

Originally it was just "Sorry, but pet peeve. Shutdown is not a verb." and I gave them the benefit of the doubt that they weren't just quibbling over a space.

[robbrown451]

True I edited, but the link to "not a verb" is pretty obvious what they are talking about. Nobody is going to make a site to claim that you can't say "shut down" as a verb phrase, that makes no sense.