How to get current registration values in node template

In the latest version of the module 3.4.2 the field formatters have options for showing "reason text" if registration is not available. So you shouldn't require a special template to get some basic text output. See if that helps.

This works perfectly, thank you @john.oltman for adding the feature and for helping. One last question: I could replace the "closed." cause with a more detailed text in the translation interface, is this the recommended way to adapt...?

You would probably have to override the field formatter in a custom module. Or try out String Overrides:

https://www.drupal.org/project/stringoverrides →

Unfortunately it does not have a D11 release yet.