As far as I am concerned, DIG is developed and published by Paradox Interactive, and so is FTM (and its subsequent patches).
I bought the DLC on Steam, and I just double checked, the information above is exactly how it appears on the "Hearts of Iron III Semper Fi: Dies Irae Götterdämmerung" Store page. One may notice that nowhere it's said that it shouldn't be compatible with the latest version of the base game, aka FTM.
Therefore, as much as I may have TONS of sympathy, admiration, genuine gratitude and patience when it comes to modders that accept to share their fine work for free with the public, as soon as they go P2P, I enter a whole another world of interaction with them : we talk business, and only about that.
I am sorry, but my € don't auto-magically appear in my pocket because I somehow managed to convince my boss to wait for 1 whole year before my work was done. One is either serious with their job, or one is not, and I wish to know where the DEV of this mod stands.
Otherwise, I may just want to look elsewhere next time before buying/recommending a game (or its DLCs) to someone, as I've learned to do with EA/Bioware (and others).
there is a big FTM patch incoming that DD is working on (3.06) that completely rebuilds the map and this is likely why the patch isn't "official"
as to continued support, i wll use DiG as an example, DD had no official expectation on him to make it compatible with FTM, he did that off his own back for no extra pay so i would say extra support is rather likely once the patch arrives