It reads to me as if AWS are now sponsoring our site. Perhaps we should contact them to find out the implications.
After looking around a bit—no, no-one from SE bothered to tell any of us—I found a post on Meta SE about site sponsorships, which shorn of ballyhoo, says:—
A sponsorship generally entails enabling ads relevant to the subject
and affixing a small "sponsored by..." logo in the upper-right corner
We're assured that:—
[...] sponsors do not own these Q&A sites. Sponsors work alongside our
communities who ultimately build these sites. Communities ask the
questions; communities create the tags; communities conduct elections
as they do now, and we are not renaming our current sites like a
garish sport stadium to the highest bidder. Any ads a sponsor submits
still have go through our crazy-strict [sic] ad editorial process… as
it has always been. Companies do not have access to personal data, and
all Q&A content remains irrevocably licensed under Creative Commons
for sharing and attribution.
To a casual visitor our site now looks like this:
This represents a considerable advance over SE's home-grown efforts at uglification, though I've not yet come across any AWS adverts flashing or beeping at me.