To replicate* Word's Ctrl+Backspace, do the following:
Ctrl+Shift+Left followed by either two Backspaces or two Deletes.
The second Backspace or Delete is not required if there is a space immediately to the left of the cursor as that space will be included in the selection.
When deleting multiple words, it is best to select them first with multiple Ctrl+Shift+Lefts, and then delete the words.
* Note that Excel's shortcut does not perform exactly the same as Word's. Word is smart enough to always delete all the appropriate spaces, leaving just one, no matter where the cursor is. The Excel shortcut, on the other hand, just selects (and deletes) all the characters from the cursor to the start of the word to the left, strictly only deleting any intervening spaces.
The equivalent sequence for Word's Ctrl+Delete is, of course:
Ctrl+Shift+Right followed by either Backspace or Delete.
Note that only one Backspace or Delete is required if there is a non-space character immediately to the right of the cursor. Otherwise, the Ctrl+Shift+Right sequence needs to be repeated, as the first sequence only selects the space.
Caveat:
Will not work unless a cell is double-clicked, F2 is pressed, or the formula bar is clicked before entering the text. This can be worked-around after text entry has begun by clicking in the formula bar at the top of the screen.