I agree! From my own experience, it was one of the things that I read about so thought it was important to give them once in a while, but the bucket it was in always got slimy. Maybe that slime was beneficial, I have no idea. It’s easy to get sucked into the supplement vortex. It’s important to remember that industry is pretty much unregulated.There are a lot of varying opinions about ACV. It's very controversial around here. You must make your own decisions.
I believe in fresh food and fresh water. My birds have access to pasture so I’m sure they get plenty of biotics, pro and otherwise. I limit treats to greens from the garden, a few meat scraps now and then and enough scratch to keep them happy to see me!
Back to the vinegar thing…I quit bothering with it several years ago and won’t bother again unless there’s a good reason.