Do I have to provide an email for a local user on Windows 10 Pro?

It's possible (I have a couple non-domain users, good "insurance").

Process is pretty simple, once you've seen it once:

• Open Settings->Accounts
• Under "Accounts" -> click on "Family and Other People"
• Under "Other People" -> click on "Add Someone Else to this PC"
• The dialog changes, click on "I don't have this person's sign-in information"
• The dialog changes again, click on "Add a user without a Microsoft Account"
• Add the username/password credentials

That's it, you're done, the Setting page should take you back to "User Management(Family and other People)" now, and the non-domain account will be under the "Other People" section.

Existing Microsoft KB here.

Or you can just open up an elevated command prompt (or powershell shell) and add the user with the following command:

net user <username> /add *

for example:

net user gizmo /add *

and type the password twice, done.

If you want to make the user an Administrator too, subsequently use this command:

net localgroup administrators <username> /add

Then if you want to get rid of the user again use

net user <username> /delete

Press Windows-R, or open a Command Prompt, and type:

mmc compmgmt.msc

and click OK/press return. This will open the Computer Management console and you can use this to manage local users.

Alternatively just run mmc, click File, Add/Remove snap-in and add the Local Users and Groups snap-in. This is the same snap-in available in the Computer Management console. You can save this console for future reuse if you think you're likely to need it again.