Ah damn, I was wrong with my interpretation.
The problem is not that the setting is disabled.
The problem seems that project templates in the workspace are only indexed when Eclipse is restarted / starts. In other words: When you create a new template project, you need to restart eclipse to use it.
I think the logic is in org.bndtools.core.templating.repobased.ReposTemplateLoader.findTemplates(String, Reporter) which is only triggered at eclipse startup. But maybe needs to be triggered when
a new template project is created
or when the "File / New / Bnd Osgi Project" wizard is opened.