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|I have grown mimosa trees in a garden with deer and the deer never bothered my trees, large or small, not even seedlings. Rabbits and other animals did not eat them, either. I always thought this was probably because the trees smell a bit like turpentine or at least quite unpleasantly aromatic when the foliage or bark is damaged. Deer in different areas will eat different plants -- they learn to eat new things all the time -- so that is not to say they definitely would not have bothered yours.
But the pattern of damage does not sound like deer to me. Deer usually nibble the tips of soft growth and leave thicker branches alone. They do not strip the branches off the tree, they just munch the tenderest parts and thus denude the branches. They like foliage and tender twigs, not woody sticks -- and at this time of year they are still relatively picky about what they eat because there is so much available. (By midwinter they are a little less selective.) Mimosa branches are so flexible and springy, they rarely break when pulled on but flex instead.
So I am really perplexed by the idea that they would have just suddenly fallen off.
You might take a careful look and see if by chance they were cut, or if somehow they were in fact intentionally torn or broken off. You might take a photo or two of the overall tree, the pattern of the damage and close ups at the breaks and consult with your county extension or a professionally trained nurseryman if you are not certain.
I am sure the damage looks just awful right now. There is some comfort in the idea that mimosas can be difficult to kill once they are well rooted. Often a tree that is cut down or severely winter damaged for example will regrow from the roots. Now if your tree was grafted and the graft has been damaged this may not work out well for you, but otherwise it may not be as much of a disaster as you think. Be very patient with your tree next spring to see if it can come back for you. Mimosas come into growth very late in the season, so wait well into June.
Good luck and I am sorry about your tree.