What is the keyboard shortcut for Keep Text Only in Outlook?

I figured it out. It isn't Ctrl+T, it is Ctrl, T (as in press and release Ctrl, then press T).

So, the pattern to paste as text only is: Ctrl+V, Ctrl, T.

Another option is Alt+H+V+T (i.e., keeping Alt pressed until after you press T).

There is an option with a small number of keystrokes, which does not involve an intermediate paste-with-format:

Ctrl+Alt+V, Unformatted text

Taken from an excellent summary of shortcuts.