There's an account setting somewhere to always send plain text emails and do not send HTML. I don't know where the setting is, I enabled it many years ago. Been happy with that.

This appears to be untrue. I looked at all the amazon emails in my inbox and they are all:

  Content-Type: text/plain; charset=utf-8
  Content-Transfer-Encoding: quoted-printable

Did you actually scroll through all the way? Some emails send both plain text and HTML. I looked through mine and indeed saw text/html for most Amazon emails (including AWS). Some AWS emails like ACM cert renewals were text/plain. Though like someone else said here, you may have changed a setting in your account preferences. The default is still HTML.

    ------=_Part_4161549_398020174.1685704999675
    Content-Type: text/html; charset=utf-8
    Content-Transfer-Encoding: 7bit

I have zero html attachments.

(These emails are not marketing emails which I don't get, they all relate to orders and shipping)

They aren’t talking about HTML attachments. HTML email is typically sent as a multipart/alternative email with one text/plain component and one text/html component. An email with attachments is a multipart/mixed email.

Mine have the text/plain header. I must have checked that setting like the sibling commenter said.

Tracking pixel in an email? I thought nobody had done that in decades because major webmail clients pre-cache all images to prevent leaking user IP etc.

I don't know about decades. It's true that a lot of clients do that nowadays, but they're definitely still used.