A pocket is a poor spot for a whistle for the chaff reason alone. But, I have found that a whistle for me in the early days with a pointing dog was too often a crutch, a stab at staying connected rather than trusting the dog and it kept me more worried than calm.
One learns, or should.
There is little doubt tho that a whistle, wisely or poorly blown, will alert birds and will also occassion some to depart....any wild bird.
But there are so many factors that will do the same, let alone one being unlikley to sneak up on a healthy bird, that the whistle of any shape is only an issue, like a voice, raised in some extreme.
Pen-raised birds are odds-on less effected by any outside stimulus and a poor comparison to wild bird reactions.
Basically, when the whistle is blown for a purpose then that purpose most often trumps any negative effect to a few birds....sometimes, by a great measure.
Much has changed from older day birdhunting, here's hoping that electronic connections twixt man and dog never totally supplant the good old whistle.
As an aside...want a good lanyard, look Knotsmith.