It's perfectly possible. Two tables, one stores answer responses only, the other just marks off who has responded. No link between them and you have anonymous data but can tell who hasn't responded.
Of course if you record created/updated timestamps on both, insert both records in the same order, accidently record the user code in the response data, take backups in between responses, have identifying questions or just don't have that many people responding it's easy/not hard to reverse engineer.
But it's quite possible to do right, I did it quite effectively almost by mistake years ago. Sent a customer survey out with generated codes as identifiers recorded with answers. Before sending reminder emails a script grabbed the codes, marked the customer as responded and wiped the code (so I could just get future responses where code was not null to mark next people off). Although I had timestamps the script meant customers were updated in blocks, there really wasn't any data to link them.
I know because the Boss was not happy he couldn't find out which customer had said what, and I had to point out all the communication (with customers and me) called it an anonymous survey, so why would I have saved them?
So it is possible, just not easy even if you intend it, and it's often not intentional...
If the participant has to trust the survey creator, then it is not anonymous. The survey creator can link the data.
If the survey creator has to trust the participant, the survey is anonymous. The participant can lie in the survey, lie about participating, or submit the survey multiple times.
Your example was not anonymous. But you did not break the participant's trust, thank you! (Or maybe you are lying.)
Anonymous example:
Sending a clean link to people to take the survey.
If not enough answers have been received, a reminder can be sent to all, with a clause, that says: "if you have already done it, you can ignore the reminder."
Of course if you record created/updated timestamps on both, insert both records in the same order, accidently record the user code in the response data, take backups in between responses, have identifying questions or just don't have that many people responding it's easy/not hard to reverse engineer.
But it's quite possible to do right, I did it quite effectively almost by mistake years ago. Sent a customer survey out with generated codes as identifiers recorded with answers. Before sending reminder emails a script grabbed the codes, marked the customer as responded and wiped the code (so I could just get future responses where code was not null to mark next people off). Although I had timestamps the script meant customers were updated in blocks, there really wasn't any data to link them.
I know because the Boss was not happy he couldn't find out which customer had said what, and I had to point out all the communication (with customers and me) called it an anonymous survey, so why would I have saved them?
So it is possible, just not easy even if you intend it, and it's often not intentional...
I don't trust anonymous surveys either now...