Tor Browser straight up does not even modify navigator.platform at all, so it's incredibly easy to see when you're not using e.g. Windows.

I believe they used to make the user-agent appear to belong to Windows but then they stopped doing it with the excuse that there are other ways to tell anyway.

The OS doesn't really matter, the amount of entropy it contains is very low. As long as the anonymity set of browser-users is large it's all good. And I believe Tor Browser accomplishes this objective.