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There is actually.... in Photoshop but I don't see any reason Illustrator won't have it. There's no one shortcut to achieve grayscale.

It's just that I don't see how  it could have happened considering you have to hit a sequence of shortcuts (3 I think).  Unless you so happen to unwittingly execute the sequence at the most unexpected moments during your workflow.....

edit....

Here's the PS version .... just in case it might work in Illustrator.

Cmd+J ... Shift+Cmd+U ... Shift+Opt+C

or

Shift+Cmd+Opt+N ... Cmd+Backspace ... Shift+Opt+C

Note: The above is what I remember for Mac.  I never used it on a PC as I prefer the long way to do it as it gives better results.