If you're marketing deer whistles, of course they work, and you can provide all sorts of testimonials from sheriff's departments about how they haven't had a single deer strike since installing whistles on all their vehicles.
If you're a scientist, studying deer physiology and behavior, you might note that the hearing range of deer is about the same as for humans. So, if you can't hear the high frequency sound, neither can a deer. And you might note that the installation instructions for deer alerts call for mounting two at least 4 feet apart, which might look rather silly on a bike. You might also note that to work properly, the whistles must have clean orifices, and if a bug happens to splot right into the orifice you wouldn't know the whistle had stopped making noise.
None of the safety organizations and insurance companies who have studied deer strikes confirm that deer whistles do anything (other than turn a profit for the salesman).
However, motorcycle safety is something of a black art. It's entirely possible that how you feel about a situation will change the outcome. That's why we have Gremlin Bells and other such talismans. When you are crossing Fairy Bridge between Douglas and Castletown on the Isle of Man, you are advised to shout, "Good day to you, fairies" to avoid some consequent flummery.
As Ron Ayres (Iron Butt rider, author, tour operator) put it to me once, "I know deer whistles don't work, but I just feel better having them on the bike."
So, follow your heart.
However, I kinda like the idea of blasting the horns while I'm hard on the brakes. At least it's a test to see if the horns are still working.
I also had an encounter while driving my yellow Can Am Spyder. A buck had leaped across the road in front of me, and had turned to leap back. When he saw the two beady "eyes" on the Spyder, he absolutely froze in his tracks. Of course, maybe he was just curious about the Spyder.