The list absolutely takes some speculation as concrete cut content. I know this because some of it is stuff that I speculated about and never suggested was proof of anything being cut, merely what could have been based on the evidence we had available. The stuff about the 'down by the river' being datamined is not true, as far as I know.

My understanding of the "truthfulness" of this list is as follows.

1. Seemingly true. There's enough oddities in the lower city (Gortash's coronation being in a prison, Cazador's mansion being through a tower rampart, the Emperor's upper city warning, etc) that definitely make it feel like there were significant changes, as do comments by Larian leading up to release. I believe people have mentioned uncovering triggers and scripting relating to the upper city from the release files.

2. Probably true. Probably cut because it made reactivity during Shadowheart's quest an issue.

3. Not true. I've seen nothing to indicate Karlach had a 'good ending' planned, much less implemented. It's more likely that the dissonance comes from Karlach being the only character to remain in the 'moodier' version of BG3's narrative, and her general vibe of being someone they only sorted out late in development (as confirmed by the artbook and changes throughout EA.) See also: how weird her 'fixing the engine' cutscenes feel.

4. Seemingly true. Unclear why cut, but I can't imagine a pregnancy plot being anything but weird (just like Aerie's plot in BG2.)

5. Seemingly true. But neglects to mention Halsin's datamined role in the Nightsong plotline (he killed Isobel with Sorrow.) Likely cut when he was made into a full companion.

6. Probably not true -- sort of. I believe datamining did indicate that Raphael would be able to remove your tadpole toward the end of Act 1, and while the soul coins were obviously supposed to be used for something other than a buff for Karlach, I don't think anyone ever figured out what they were for. I believe there was a suggestion during EA that they'd be used to create hirelings. There was datamined content that suggested Raphael would have generally bigger role to play, however.

7. Almost certainly not true. I don't think anything Avernus related was ever datamined.

8. Kinda-sorta. It appears that Tav was something like a Bhaalspawn in EA, given some of the responses they'd have to Daisy. It is unlikely, however, that they were the Dark Urge. That said, it does appear there was a distinct split between Bhaalspawn!Tav and Release!Tav during development, and it's likely the Dark Urge was constructed as a result of that split.

9. True. Likely altered when Daisy was.

10. Seemingly true. Lots of content was turned up about this, and elements of it still persist in game (injured Voss, for example) and the general oddity of the Underdark and Mountain Pass being presented as equivalent when one is much larger than the other. Additionally, plot-related oddities in the creche sequence may be evidence of this.

11. Seemingly true and directly related to point 10. A Magic: The Gathering card hinted at this.

12. Not sure.

13. Seemingly true. Larian apparently cut these are they thought they were too long. Yeah.

14. Seemingly true.

15. Wrong. Banite cultists do feature in the game, although they do feel limited and inconsequential.

16. Apparently true.

17. True.

18. True. Similar to Shar, it was likely too problematic from a writing perspective.

19. True, but this runs with it a little. It's clear that Daisy and the Guardian/Emperor were vastly different implementations. It is likely that Daisy was the Absolute attempting to give you power to use against the Chosen keeping it enslaved. Datamining suggested there were two Daisies and one of their aspects was that of Orpheus. It is highly likely that the songs mentioned did relate to this older plotline. But I've seen nothing to suggest that ending was ever anything more than something I speculated upon a few days before this post went up.

20. True. The companions used to be visited by unique Daisies (Vlaakith for Lae'zel, Cazador for Astarion, Mystra for Gale, etc.) and this has been removed. Similar to Daisy, it's likely that these changes were a result of whatever happened behind the scenes concerning Daisy, the illithid plot, the desire to give players cool powers, significant changes in tone and narrative, and whatever else.

21. Unsure. Leaning toward no, given that she had a full-fledged romance arc in BG2.

22. True.

23. Potentially true. Given the state of Act 3, it's unclear what is not present versus bugged out.

24. True.

25. True. That said, cutting the nautiloid down makes a lot of sense given how often you might need to run through it. Irenicus' Dungeon Begone, anyone? However, it is notable for eliminating one of the few scenes featuring Mind Flayer thralls.

26. No idea. Given you can find Mol's infernal contract and how oddly it's handled, it wouldn't surprise me if there was more intended (see also: Raphael.)

27. No idea.

28. No idea.

29. See 13.

30. True. That said, I'm pretty sure this is an idea that never really got close to being implemented.

31. True. Likely cut as some consequence of Halsin's popularity. Suggestions have also been made that her werewolf aspect was rolled into the Dark Urge.

31. Is there a belt slot in the game?