The Stammer Jammer - noise played when talking

This is the 'Stammer Jammer', which was invented over 20 years ago (maybe 30 years ago), and was a simple device that used a microphone, and headphones to play a noise into your ear, whenever you started to talk.
On the market today (2023) there are three or four devices which play a delayed and higher pitch version of your own voice into your ear. I have been looking at the reviews of these devices, and none of them seem to work for the majority of people, the majority of the time.

The Stammer Jammer, on the other hand, seemed to work for people so well, that after a length of time using it, they became fluent and didn't have to use the Stammer Jammer any more. Why do none of the devices on the market play a noise into your ear when you start talking, instead of merely delaying your voice and raising the pitch? Has no research been done into finding the most effective thing to play into your ear, when talking, to stop you from stuttering?

You can now buy a basic hearing aid on Ebay for £15. I imagine the extra technology to add a microphone, and play a burst of noise into your ear at the start of every word, would be less than £5 extra. So why are some anti-stammering devices around $4,000?

Does anybody have links to any research done by universities, etc. on anti-stammering devices such as the Stammer Jammer? You would think it would have been done by many speech therapists and other interested doctors/therapists.