In older versions of Windows and even in Windows 10 previews you could enable the built-in guest account in control panel
Control Panel\User Accounts\User Accounts\Manage Accounts
In Windows 10, the
Guest entry is missing.
Computer Management or
net user you can still enable the guest account, but even then it doesn't show up on the logon screen.
In the Local Security Policy
Local Policies - User Rights Assignment - Deny log on locally
we can see
guest listed, this wasn't the case in older versions of Windows.
guest from that policy, it is listed on the logon screen and we can using it to log on.
However, the whole Explorer shell is crashing and restarting constantly. The desktop is blinking and you can't click anything.
So the guest account is totally broken.
There is a question on answers.microsoft.com about this showing that some code was quickly added to the control panel to not show the guest account. So Microsoft does know about the problem.
While I don't really expect anybody to know what they've done to break this, at least this explains why the guest account is currently not listed.