Each section covers the upgrade from the previous LTS release, the section on 2.401.1 covers the upgrade from 2.387.3.
No notable changes requiring upgrade notes.
A custom JavaScript library, jenkins-js-modules has been removed from Jenkins core.
Users of the OWASP Dependency Track plugin must upgrade to 4.3.1 or newer.
Users of the ServiceNow CI/CD plugin must upgrade to 2.1 or newer.
The default connection mode for the Java command line interface client is now -webSocket.
You can specify -http to continue to use the former default (for example because you are running Jenkins in a servlet container other than the recommended built-in Jetty, or because you are running an unusual reverse proxy which does not support WebSocket). You can also continue to specify -ssh to use SSH transport (for example because you prefer to authenticate with a private key rather than an API token), or use a native SSH client.
If you use the Jenkins CLI but cannot make WebSocket connections to the Jenkins controller, you will now need to pass the -http or -ssh option if you were not already doing so.