...
Starting with version 2.4.0 of the IdP, there is an easier way to add content to the default Velocity template templates that generates generate the POST response web page that is returned to the user's browser, and then auto-submitted to the appropriate SP endpoint. One has always been able to override those Velocity templates altogether, and create your own templates, if you know what you are doing. But this new feature for the IdP makes it much easier to make particular kinds of additions to those pages without needing to completely override the default pages.
...