It is a thing. These "Other" gods are just monsters you can kill.
Not like the one Gygax's family and community worshipped. THAT one can't be killed. And putting 'him' into the game would've resulted with backlash from people who might actually be a danger to Gygax and his buds.
As far as I know Gygax didn't write or have any real involvement in the creation of Deities & Demigods beyond writing a Foreword.
The idea in 1980's America (and I am not American so I can only guess) was presumably...let's not offend the majority of people living in
this country where we sell 90% of these books by having stats for "GOD". Instead lets use figures from
mythology and characters from
mythology books.
But the gods of foreign cultures and dead cultures? Fair game for the game.
I don't see a problem with using characters from
mythology in any capacity.
So really your criticism is presumably based on the section on Hindu deities - or perhaps the Ancestral Spirits of the Native Americans? Maybe the Norse gods are having a revival - but I doubt you criticize Disney for their appropriation of those deities in Comic, Animated and Movie formats.
Not sure how familiar you are with Bollywood movies and television shows (I am) but India really seems to love dramatic depictions of its immortals in animated and live action format. I have also spoken to a few Indian roleplayers/DMs and none of the people I have gamed with or chatted to (at various London D&D meet-ups) have any problem with depictions of their deities.
I have not spoken to any Native Americans on the topic of whether they have a problem with the depictions of their gods in Deities & Demigods or similar books. However, I do possess maybe 50-100 books on
mythology and these Native American spirits/deities are discussed, depicted and expounded upon within many of these books. A quick search on Amazon reveals there are over 1000 books on "Native American
Mythology", so it seems like there is no one protesting Amazon about the matter.
Care to explain why you seem to be upset about the depiction of
mythological figures in D&D?
Same reason you can find the stats for Bane, Kiaransalee, and Mystra but not Ao, who is so powerful game stats don't represent him at all.
I'd have stats for AO. There is a place for Monotheistic 'Overgods'. The Tier above the Immortal Tier seems to work fine.
However, you'll notice that while PC levels cap at 20 the Challenges in the Monster Manual go up to 30. Thus stats for Orcus and Tiamat have a purpose as challenges for groups of Level 20 characters. Stats for AO serve no purpose unless PCs can themselves become immortal (for which there are currently no official rules).
Thus even back in 1980, there was mechanically no point putting Monotheistic Gods into Deities & Demigods since they would logically have to be beyond Greater Gods and there were no rules for PC's becoming Gods (the suggestions for Ascension in D&Dg led to Character retirement from the game) to put PCs in a place where they could challenge Overgods.
I mean I suppose they could have just had an Overgod rank 'for fun' with 1000 Hit Points (as our AD&D campaign did back in the 90's) - though Overgods themselves never saw play; even if many Greater Gods (and Monsters with 400+ hp ) did in our very high level game where PCs could play as Gods.
Because what you exclude from your fantasy games is just as important as what you include for the whole "Art is Politics" thing.
I very much doubt you are offended by anything that was either depicted, or not depicted, within Deities & Demigods.