Questions regarding "Shortening the Let's Encrypt Chain of Trust"

The "short chain" or "alternate chain" has been available since May 4th, 2021: Production Chain Changes

As of today, you need to manually request this chain via your ACME client (it's typically called the "preferred chain" feature). The details depend on your ACME client.

There isn't anything special about the certificates. You can serve every Let's Encrypt certificate with the long or short chain. It's just a matter of how your webserver and/or ACME client is configured.

Also see Long (default) and Short (alternate) Certificate Chains Explained

The webserver at appears to have been (manually) configured to serve the short chain. This is not the current default.