If I fabricate a killstreak item that is not tradeable until X date, will the item become not tradeable permanently?

Having personally done this myself, applying temporarily untradeable killstreak kits to a tradeable weapon transfers the trade date. Once the 7 days passes, your killstreak weapon is once again tradeable.

As Aaron mentioned, this wasn't always the case, but was patched during 2014's Smissmas update.

Correct me if I'm wrong here, but the December 22, 2014 patch had fixed this:

Crafting items that are marked as temporarily untradable will now yield temporarily untradable items instead of permanent

and Killstreak kits function the same way as crafting, so it would be reasonable to assume that this applies to killstreak kits as well.

A quick search on google will show no more questions about killstreak kits or other items being perma-uncraftable in a situation like this after December 22, the day of the patch.

Much of what has been said seems to imply that it retains the timer, but to be safe, just wait the seven days. It's always better safe than sorry.