There’s also this I just found via search merge user accounts. This is still available by clicking on the user in the active user list (Admin Dashboard) and scrolling down to the bottom to see the 3 buttons shown below.
Discourse 2.5.0.beta3 Release Notes
Merge Users via the UI
In early 2018 we added support for merging two user accounts into each other. To do so required SSH access to the server, as merge support was added as a rake task.
Users can now be merged by any admin! Visit the user admin page, and select “Transfer & Delete” at the bottom of the page.
Enter the username you want the account to be merged into, and click “Transfer & Delete @username” to start the merge process.