Each section covers the upgrade from the previous LTS release, the section on 2.89.1 covers the upgrade from 2.73.3.
The launch method Launch agent via execution of command on the master has been moved to a new Command Launcher plugin and integrated with the Script Security plugin.
Two potential problems have been identified with this change:
Jenkins does not automatically install the plugin when upgrading from before Jenkins 2.86 to that version or newer. This includes an upgrade to 2.89.1 from any earlier LTS release. The plugin will need to be installed manually after an update. If it appears to be unavailable on the update center, make sure to update the cached metadata by clicking Check Now.
The integration with Script Security can result in problems with plugins making use of this launch method internally in a way that is incompatible with the security model of Script Security. A known instance of this problem is EC2 Plugin. The issue is tracked as JENKINS-47593.