Question such as Can I use a PC license to play the Mac version? become useful once the game is released. Should it be reopen automatically or let the user to flag for moderator's attention and ask for reopen?
Speculation about unreleased games is off-topic as per the FAQ. Debating what constitutes speculation in regard to official developer/publisher statement can be left to another meta post. For the sake of this answer, let's assume the questions it applies to are properly closed.
Should we reopen questions about unreleased games once the game is released?
This would encourage people to ask questions about unreleased games, have them closed, and then they get all the rep when the question is reopened. This is called "parking" or "camping" and should not be allowed.
But what if the question is not an attempt to park and was just asked by a new user?
That's understandable. Direct them to the FAQ, meta, or chat. The question still should not be reopened. While new users shouldn't be immediately punished for their ignorance, they should not be rewarded for it either. Furthermore, users who follow the rules by waiting until the game is released to ask their questions should not be punished for those actions. Reopening such questions would do exactly that. Reward the rule breakers and punish the rule followers.
Instead, politely leave a comment explaining the current policy and encourage them to learn other policies by reading the FAQ and meta. We should be helping people become better users, but we don't have to leave rules broken to do so.
I believe the better course of action is to delete the question for now and re-ask it, if necessary, when the game is released.
The major problem I see with questions about the content of unreleased games is not that they lead to speculation, though that is part of it. But there is also no real point to asking them before the game is released, they are not actually useful, and they tend to get much better answers once they are released.
But there are certain types of questions that are useful before the game is released, as they help you decide on how to buy the game, or if you want to buy it at all. I don't mean questions that help you decide whether you'd like the game, those aren't a good fit. If you want to know about the game before it is released, read some previews. If you're still unsure, just wait until it is released and you can read a larger number of actual reviews.
The types of questions I'm thinking about are for example
Essentially, questions that determine on how to buy the game do seem useful to me, and while there might be no definite answer to them, this kind of information is something that the publishers tend to make available.