Nah, they are trades. No rule against putting up a trade you will execute in the future. In fact all trades are actually done in the future, however far or short into it.
And the fact that it is known that they will become tradable lets you assume that those early trades indeed have the intention to trade their tux.
And it's probably not a bad idea either if you intent to sell, as I suspect they will fall in value quickly once they become tradable.

I don't see why they should be closed as the rules only state that you shouldn't create trades for items you aren't going to trade. Just because it's untradable now it doesn't mean you don't want to trade it when it becomes tradable so why not get offers on it while you wait.

Originally posted by Uranium235:

And the fact that it is known that they will become tradable lets you assume that those early trades indeed have the intention to trade their tux.

Is there such a fact that they will become tradable?

No

Originally posted by Not_Duck:

No

I would say yes

Originally posted by Steam News:

Steam users who play the game on Linux will automatically receive a free, exclusive in-game item: Tux, the Linux mascot, can be carried by any of the game’s classes and traded between players.

Source

no