Redesigned login, signup, and Jenkins is (re)starting pages. Existing page decorators like Simple Theme Plugin will no longer work with these redesigned pages.
(issue 50447, announcement blog post)
The deprecated Jenkins CLI Protocol versions 1 and 2, and Java Web Start Agent Protocol versions 1, 2, and 3 have been disabled. If you still use these protocols (e.g. remoting-based CLI, or old agent.jars on agents), you need to re-enable these protocols after upgrade, or upgrade the clients. The same recommendations as in The 2.121.x upgrade guide for remoting changes apply here.
(issue 48480)
Jenkins remote API: Export fingerprints for builds which do not derive from AbstractBuild, like Pipeline builds.
(issue 51667)
Stop using deprecated com.google.common.io.NullOutputStream from Guava to avoid binary conflicts with plugins bundling newer Guava versions (regression in 2.127).
(issue 51889)
Developer: Introduce SimplePageDecorator extension point, which allows decorating the redesigned login page.
(announcement blog post)
Developer API: Prevent NullPointerException in SlaveComputer#setChannel(Channel,OutputStream,Channel.Listener) with nullOutputStream.
(issue 51955)
Developer API: StreamTaskListener#getCharset() now returns the default charset when it is not configured.
(issue 51971)