in version 2.3.1 the user approval register in adminstration/user/… is missing.
If I’m understanding correctly, you are trying to find where user approvals are handled?
If so, this has been moved to the Review page which you can access from the hamburger menu. You can choose filters using the dropdowns on the right of the page then select the refresh button to see the results.
If you want a direct link to see the full history of user approvals, you can go to:
/review?status=all&type=ReviewableUser.
You should get notifications for new user approvals which will take you right to the approval record when selected.
Let me know if I’ve misunderstood the problem 
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