Is my Apple Id under attack, and how do I fix it?
I would absolutely follow the “if you did not request this” instructions to the letter. Since you may have difficulty with security questions or past purchases, I would open a support case with Apple and take good notes what you trying anything seems odd.

Just be sure you don’t click the links in the mail - it’s getting hard to tell the scam mails from the real ones if you just look at font and spelling and not the address that links open. 

Navigate to apple support site from https://apple.com

The main problem is if you don’t keep good records of all your security information when you signed up, you might need to be patient and dig up app purchases, review old credit cards systematically and work with support. 

A typo in a year is something a computer won't validate and you know the year you were born, just that might not be what the keyboard typed. Worse, if you made up answers or answered the correct question in the wrong field (answer in the field above the question instead of below the question), you could easily lock yourself out.

That ends the thing you control, here are things you don't control:

There’s no way you can prevent someone else from trying to lock your account initially or even take it over. Until you validate to Apple support that you are the owner of the account, the account could be "flagged" for extra security. Hopefully it is just someone that is confused and not someone maliciously trying to make your life miserable.