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And almost all physical copies of games require an activation on a platform like Steam, UPlay, Origin etc. anyway.
The only difference would be the presence of data you can install from DVD, which most likely is already outdated and/or incomplete.
Does this rule include all games released even before Steam became a thing? Let's say back then, I had a physical copy of an offline game which I can install on anyone's PC. Then today, I had to install it again but I lost my copy. Now, I can only download it through Steam. If I purchase it again on Steam, is there no way I can install it on anyone's PC without the need for Steam?
There are only 'few' games on steam you can play without having steam running in the background but those are usually the ones that wasn't well implemented into steam and basically only an "import" of the original version.
No. Games used to be "linked" to a physical media, usually by means of a "copy protection" mechanism that was meant to ensure that the original disk would be accepted, while rejecting copies.
Nowadays, games are linked to an online account such as your Steam account. In order to play the game, you don't insert your disk into the drive anymore; instead, you insert your accountname/password into the Steam login window.
No, you can't transfer ownership of a product on steam.