Salesforce added Duplicate Rules and Lightning Experience is missing Merge Accounts and Contacts in tools. If you would like to the merge tool just like in classic. You can add custom button to list views. Here is how you can do it:
- Create a custom button. Display Type – List Button, Content Source: URL and use this URLs :
2. Add button to the List View Search Layout.
To add button navigate to Object Manager => Account => Search Layouts for Salesforce Classic.
Edit List View layout and add Merge Accounts button.
Now if you you go to any Account List view you will see Merge Accounts button.