As someone who has worked freelance and as a sub-contractor online for years, anyone who may potentially send me a paycheck has to, by law, have your social security number with this. This is so the CRA (or in my case IRS) can compile the earnings of a person and make sure certain things are claimed. When I sign up for sites I have crowdfunded on or receive donations on this process is commonplace. Should you earn enough that it's required you file taxes that year you file freelance work on a different form since taxes aren't taken out of it by the employer but rather it's the freelancer's responsibility.
A good thing to keep in mind with that is that should these earnings be substantial set aside 30% received in a bank account so that if all your income is freelance that is the amount you might need with taxes. In the US the form for freelance is 1099, not sure what the Canadian equivalent is but you are totally fine. Itch.io couldn't do anything otherwise it would be a huge federal offense so no worries.