The Registration app is no longer necessary. It has been removed from the Atlassian Marketplace in spring 2020. You can uninstall the app in your instance.
The Registration app was used to register the following free bitvoodoo apps for Server&Data Center:
Language Macros for Confluence (now Translations for Confluence, a paid app)
External Links for Confluence (now a paid app)
SBB Widgets for Confluence (now in Lab status)
Macro Documentation for Confluence (now in Lab status)
If your company has ever used one of the apps listed above, chances are that the app “bitvoodoo Registration for Confluence” is still listed in the “Manage Apps” list of your instance. This app is no longer necessary and you can safely uninstall it.
If you are migrating bitvoodoo apps to Confluence Cloud, make sure to uninstall “bitvoodoo Registration for Confluence”.