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Steam never gives us your password or email. What does it share?

You sign in on Steam's own site: check that the address bar says steamcommunity.com. We never see your Steam password or Steam Guard code.
Steam tells us one thing: your Steam ID, the number in your public profile's address. No email, friends list, games, inventory or payment details, and no way to act on your Steam account.
If no forum account is linked to it yet, we read your public Steam name, to suggest it as your username, and check that the account isn't a limited one. We also look your Steam ID up in mR's 2008-2010 server records, to find your old forum account.
If you don't go on to make an account, nothing from Steam is kept: the unfinished sign-in is thrown away within two hours. If you ask the admins for an old account, they keep your request (your Steam ID and name, and what you wrote).
If you do, your Steam ID stays linked to your forum account so you can sign in with Steam, with a log of those sign-ins (time and IP address). Only you and the forum's admins can see it, and you can unlink it in the User CP.