There is no mention of 'Left' or 'Leaving' in the Phraseology section.
That's right and you could use either depending on whether or not you have been cleared to descend.
If you were cleared to descend, you descend, but are waiting for someone else to finish their long-winded radio call, then you will have "left' your FL. If you are in fact leaving your FL, you would say you are "leaving." Pretty simple really.
They are not the same thing.