As someone who dabbled in EA and datamining I can say without a doubt that certain things that were supposed to be in the game were cut.
Are all of those things mentioned on the cut content list things that were supposed to be in the game/cut before launch ? I doubt it, it's clear that some of them changed during the development and took on another form when Larian decided to go into another direction.
But at the same time there are things that just stand out, unfished quest/storylines etc. that other datamined info shines some light on.
When it comes to Karlach can you really say you are surprised that people are getting emotional over romances in RPGs (I still remember that post on the old BSN forums where some guy wrote a whole book about Talis sweat, people are weird man)
I do agree with you that there is no clear proof that there are any cuts in regards to her story but is there a feeling of a rushed story? I would say yes but then again I can say that about the whole game.
And the reddit part, that place is an echo chamber lol. I wouldn't take it too seriously.
Yeah, nothing about Karlach seems to suggest she has a happy ending sitting there but unimplemented. I've mentioned before that Karlach's ending is tragic but fits much better with the themes of BG3 back in EA than it does in release -- what is the worth of surviving if you sell your soul to do it? Karlach is the one character who would choose not to compromise her morality and ethics just to live longer, and, so, gets to die surrounded by her friends after having saved the day. This is really powerful! Her ending is tragic and memorable, the one bit of my BG3 ending that I can recall vividly. If anything, the issue is not that Karlach has a sad ending but that she's the only character to be forced into a sad ending.
I agree with this but only in part. My issue with her story isn't the sad ending (death or Avernus) but the lack of exploring the choices that the game gives you in trying to help her. After the second conversation with Dammon her storyline just ends despite there being other smiths/wizards/etc. in that game that could at least try even if they end in failure.