Is there a limited availability of the Digital Deluxe Edition of Heart of the Swarm?

I haven't heard of any announcement declining or supporting such a limited availability, considering it's digital only (so nothing to produce and store somewhere).

Especially considering there are skins unlocked by the DDE/CE that alre also listed for People only owning the Standard Edition (like me), I'd assume they wouldn't limit that supply, even if it's just for those "completionists" who'll buy the upgrade just to get everything.

Right now they offer the DDE upgrade for 15 € (possibly similar price tag for other regions; only available if you already own the game).

My personal opinion: That price just for some cosmetics isn't really something you should worry about. For 15 € you can get better stuff than that. If you're not interested in the physical CE, just grab the Standard Edition and play the game. Later on, if you still think you might want the DDE, grab the upgrade. Worst case (they stop offering it), you'd still have the full game, just minus that skin stuff.

I've found no mention of a limit by quantity, but there may be a limit by date. This Game Informer article suggests that you get it "while it is available" and confirms the upgrade Mario mentions, which is $20 US if your not in the EU zone.

...you can upgrade to the Digital Deluxe version for an extra $20 while it's still available.