Preface: a piece of content being in a previous expansion does not mean you are entitled to get it in every expansion moving forward.
There is no Deep Dungeon because they made Bozja instead.
The cutback on dungeons began back in Heavensward as a means to direct resources elsewhere, which has given us such content as Ocean Fishing, Deep Dungeons, and Bozja. Resources aren't unlimited.
Data retrieved from datamining isn't necessarily reflective of what's implemented. Think of it as prototype data, rather than what's "supposed to be" implemented. (I agree it feels like there's something missing in the Diamond Weapon fight and other than the armored Diamond Weapon's model find it kind of lazily done all around, but it is what it is.)
... but rather than say they're just being lazy or rushing things, I'm willing to chalk it up to dealing with the pandemic and the fact their resources are limited. (I remember an interview somewhere where Yoshi-P said the dev team is chronically underfunded and understaffed. Not sure how reflective that is of their current circumstances, but.)
I agree it feels like there was a lot cut in this expansion, but am more inclined to pin the blame on resource shortage and the pandemic than just say the devs are being lazy or negligent.