The bouncycastle API, Command Agent Launcher, Oracle Java SE Development Kit Installer, JAXB, Trilead API, SSH server, JavaBeans Activation Framework (JAF) API, JavaMail API, and Instance Identity plugins are no longer bundled inside jenkins.war, reducing its download size by more than 20 MB. This functionality was split out of Jenkins core into these plugins between 2016 and 2022 (Jenkins 2.16 through 2.356). Copies were bundled in the war since then so that instances upgrading from such old versions, or plugins built against them, would keep working.
Most instances are not affected. If the update center is reachable, Jenkins resolves and installs these plugins automatically over the network whenever something needs them, the same way it would for any other missing dependency, regardless of which Jenkins version you're running.
You are only affected if your instance has no access to the update center. In that case, upgrading core past one of the versions above, or installing a plugin built against an old baseline, can no longer pull these plugins in automatically. Download the plugin files from the update center and place them in the plugins directory of the Jenkins home directory before starting the new version, or before installing the plugin that needs them.