If I remember correctly, they've also moved around the "No thanks" over the years. I seem to remember the "No thanks" option being below the "Enjoy Prime for FREE for blah ..." area a while back. I could swear the prompt changes sometimes between this more standard screenshot above, and some weird pitch geared specifically towards college students, where the "No thanks" option is considerably harder to see.
I was trying to reproduce the prompt that I was thinking of and found an equally obnoxious prompt:
https://i.imgur.com/8HNjFHl.png
If you accidentally click on prime, you are shown this
https://i.imgur.com/hL60GFc.png
Which makes it seem like you can't even remove the "free" Prime trial, unless you look extremely closely.
Alternatively, if you do click on free shipping, but not on Prime free shipping, you get a popup showing this
https://i.imgur.com/AHX2KNg.png
Which defaults to trying to steal any gift card balance you have in order to pay for Prime.
I knew that Amazon was awful, but it's really gotten so far out of hand that it's surprising they haven't had the sort of legal trouble Microsoft had back in '98.