I'm using Pandoc version 3.6.3 (on Windows 11, 64 bit) to transform Markdown files into standalone HTML pages. I let Pandoc deduce the formats from the file extensions, my command line is like this:

pandoc source.md -o dest.html -s

The CSS style (reduced to the body section) I find embedded is the generated HTML file is like this:

    body {
      margin: 0 auto;
      max-width: 36em;
      padding-left: 50px;
      padding-right: 50px;
      padding-top: 50px;
      padding-bottom: 50px;
      hyphens: auto;
      overflow-wrap: break-word;
      text-rendering: optimizeLegibility;
      font-kerning: normal;
    }

However, in the Manual, section Variables for HTML tells us the following:

maxwidth
sets the CSS max-width property (default is 32em).

Since I found this comment by you @jgm in the issue #8577,

  * HTML template:

    + Remove default font size, line height and font family in
      default inline css (#8423). `mainfont`, `fontsize`,
      and `linestretch` can still be used as before; the only difference
      is that we no longer provide opinionated defaults.
      This commit also adds a `maxwidth` variable that sets `max-width`;
      if not set, 36em is used as a default.

So maybe we have it right in the version-3.0 changelog but wrong in the online manual.