I notice "an" is in decline in some forms of spoken English. I noticed this shift years ago. Some English speakers say "a (y)apple" rather than "an apple" or "a(y) orange" rather than "an orange". In some case the "y" sound is replaced by a glottal stop (a' apple, a' orange).

[Why is this getting voted down? Bizarre.]

Some YouTubers always say a like the letter a (e.g. the vowel in "day")

I find it pretty annoying. A different YouTuber had a good video about it though https://m.youtube.com/watch?v=4IJLyFAzYes&ra=m

a.k.a. "hard attack". There's a nice video on the subject by linguist Geoff Lindsey: https://youtu.be/nCe7Fj8-ZnQ

I've also noticed this but it sounds kind of infantile to me. It's a common pronunciation that a lot of children make seemingly by mistake and then eventually stop doing. When adults do this it sounds wrong, but I admit I have heard it.

No