The Merge Users Plugin, developed by Dannii, allowed administrators to easily merge two users together. However, as of Discourse 2.5.0 beta 3, this feature is now built into Discourse, making the plugin redundant.
Dannii recommends uninstalling the plugin as it may cause errors, especially after a recent software update. The plugin’s Github repository is still available for reference.
The plugin’s installation process involved adding a line to the app.yml file and rebuilding the container. However, with the feature now incorporated into Discourse, this is no longer necessary.
Dannii notes that the plugin’s idea being incorporated into the core software is a compliment, indicating the plugin’s usefulness.
This plugin will not be supported long term and may break soon..
Update Dec 2022: I think a recent software update has meant that the plugin causes an error when someone is not an admin:
Unhandled Promise Rejection: Error: Could not find module admin/models/admin-user imported from discourse/plugins/discourse-merge-users/discourse/components/merge-users-form
So if anyone else still has this plugin installed, now is the time to uninstall it.
Nothing bad at all about @Dannii’s excellent work here, but we have no need for this plugin anymore since it is all built into Discourse, leaving the link and topic around mainly as an educational artifact.