You cannot filter out just the merged tickets from the Tickets tab. As a workaround, you can automate adding tags to a ticket, when it is merged. To achieve this, create a new automation rule under Admin > Workflows > Automations > Ticket updates > New Rule.

When Action is performed by: 


Involves any of these events: 

Note is added > Private

On Tickets with these Properties: 

Last Interaction > contains > This ticket is merged into


Add Tag > merged_ticket

This would add the 'merged_ticket' tag on the ticket that is merged into another. After this is done, going forward, you can filter using this tag from the Tickets list page to get the list of merged tickets.