I've just discovered that "&" was considered the 27th letter of the alphabet, being part of alphabet songs.
It was easy to discover its history (the information on the website Fast Company is repeated on many other websites), but nowhere have I found a reference to why and when it was dropped out of the alphabet.
I can only guess it was dropped out because it represents a whole word rather than a sound, but I still can find no references as to when that happened.
This question isn't a duplicate of & as a letter in the alphabet? because that question is about how '&' could have been considered a letter and become part of the alphabet.
Yes, the answers to that question (and the first answer to my own question) helped me understand that ampersend was never truly considered a letter, nor was it ever a true part of the alphabet (though I suspect children may have thought of it as its 27th letter). Just... something annexed to it?
Following that new knowledge, I adjusted the question: when did '&' stop being taught alongside the alphabet?