Twitch support sent me right here without explaining why the embedding was blocked. So I’m trying to understand what exactly I violated and how to get the embed unlock.

<div class="twitch-embed-div">
   <div id="twitch-embed">
      <iframe src="https://embed.twitch.tv/?channel=xop0&height=400&migration=true&muted=true&parent=freetp.org&theme=dark&width=400" allowfullscreen="" 
      scrolling="no" allow="autoplay; fullscreen" title="Twitch" width="400" height="400">
      </iframe>
   </div>
      <script type="text/javascript">
         new Twitch.Embed( 'twitch-embed', {
            width   : 400,
            height   : 400,
            channel   : 'xop0',
            muted   : true,
            theme   : 'dark',
            parent   : ['freetp.org']
         } );
      </script>
  </div>
</div>

The site does not contain anything that violates the rules. A regular gaming site.
Perhaps because of the size, one of the parameters is less than the minimum, but I have seen many sites have smaller embeddings. For example https://www.igromania.ru/ 375x275 on every page.

And what is the minimum size if:

Embedded video windows must be at least 400x300 pixels.

Height of the rendered element, in pixels. This can be expressed as a percentage, by passing a string like 50% . Minimum: 400. Default: 480. Maximum width of the rendered element, in pixels. This can be expressed as a percentage, by passing a string like 100% . Minimum: 340. Default: 940.