Is this ok to ask questions for public demo versions of an unreleased game?
Yes, absolutely. If it's available to the general public, then it's fair game. This applies to King of Seas, since anyone can download it from the games Steam page. The only real caveat I'd put to this is it needs to be a question about something actually in the demo. This would to both game mechanics and content.
For example, the Monster Hunter Rise demo allowed you to fight a handful of different monsters. You'd be free to ask questions about any of the monsters that are in the demo, but asking questions about monsters that aren't in the demo wouldn't meet our "available to the general public" threshold.
In a similar vein, if the question was "How long does the Greatsword take to fully charge?", then that's a weapon and ability (aka a mechanic) usable in the demo, so it would also be perfectly on-topic. An example of an off-topic question would be "How long does the Bow Level 4 charge take?", since a Level 4 charge is only accessible via armor skills which weren't available in the demo.
How should it be tagged?
I think this depends as much on the question as it does on the demo.
Some demos are literally just a small little slice of the start of the game, and anyone who purchases the full game can just transfer their save data from the demo to the full game. For these kinds of demos, I don't see any value in adding a
-demo suffix. I'd say that even questions such as "What's the max level in the demo?" would be perfectly fine, since this kind of demo is really just like playing some of those "free" versions that MMOs and other games sometimes release.
As for other types of demos, I'd say it really comes down to "Is this content that's only available in the demo?" If it's demo only content, then we should use a
-demo suffix. If the content is available in the demo and the full game (such as the demo monsters in my Monster Hunter Rise example above), then I don't think a
-demo suffix is required.
So on that note, to me, game mechanics are almost always going to fall into the latter category, where they're identical in the demo and the main game, so a
-demo suffix isn't required.
Of course it can be difficult to answer the above criteria before the full game is actually released... but I honestly don't think it's a huge concern. Once the full game is released it's usually pretty obvious when it would make sense to add a
-demo suffix, so we can always retag questions after the full game releases if we realize the answers for the demo differ significantly from those of the main game.