I mean, it's not really optional for Canadians _not_ to sign up for the list. It's the official list of electors. If you're a citizen, you're going to end up on the voter list one way or another.
It actually is optional. You should always be able to opt out of registering for it, and if you are registered and don't want to be, there's an official form that you can fill out to get removed. See my other comment [0] for a few more details.
The data sharing between the CRA and Elections Canada is optional, but if you want to vote, you've got to be registered - whether via the CRA or otherwise.
> but if you want to vote, you've got to be registered - whether via the CRA or otherwise
Technically true, but you can register at the polling booth on the day of the election, and there's a checkbox that lets you opt out of saving your data in the database. [0]
[0]: https://news.ycombinator.com/item?id=48015977