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.