Friend of mine
Well, there are actually quite a few issues with this plugin. If you set your steam nick to an account name already existing on the forum, your account gets created successfully. The username:uid and userslug:uid tables, however, get updated aswell, making a click on the profile of the original account redirect to yours and making them unable to login. (See https://github.com/igorprado/nodebb-plugin-openid-steam/issues/9)
Similarly, you can't register if your steam nickname contains special characters and what if you don't want to use the steam name you have at the moment? Some people change their steam name like their underwear.
Another issue is the one @Radon mentioned earlier, where there is no way to link your steam account with your account.
Maybe add a little button to the profile? (Should be possible with filter:posts.custom_profile_info, maybe?)
And probably a similar button for unlinking (which admins should be able to turn off, I guess)