This is your contact url. If someone simply increments this number you can see everyone who has signed up (some duplicates).
Not sure if you think this is a feature that should exist.
Why is this an issue? If someone 'signed up' to see if it would work and decided not to use it - they're here for as long as their keybase account exists along with a link to their email address.
They could block the domain, but that's not a very friendly solution.
Consider adding to the roadmap some type of more random user generation number/method (if you haven't already) - or some other authenticated way to remove your account.
As suggested by a user: