I'm a citizen of Canada. I'm doing the tax interview with itch.io and got to the part asking about my TIN  (Tax information number). I looked it up and the Canadian equivalent would be my SIN, social insurance number. However, I'm a bit hesitant to give that out. Has anybody here from Canada done that or has any information that could assure me this is safe, and that my information remains private? 

Is there a way I can add monetization without having to input my SIN?

I'm from Switzerland. For the interview I had to give a photocopy of my identity card and my social security number to this organisation that makes the tax interview for itchio. I was a bit concerned about it, as you are, but so far I had no problems. I decided to trust itchio. Not that I had a choice, but I decided to trust Leafo. 

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.