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 www.worthingtonlibraries.org appears to have been (manually) configured to serve the short chain. This is not the current default.