So I might have skipped a few hundred pages somewhere in the middle, but believe or not, I've actually read most of this ludicrous thread.
Some random comments on a few of the things being discussed:
Is D&D simulationistic? I think it is somewhat, though there are a lot of gamism in it. Things tend to pay lip service to simulation, but main concerns are gamist, and 5.5 (unfortunately for me) seems to be going even more heavily into that direction. However, like already pointed out, how simulationistically it plays in practice, depends a lot on how the GM implements the rules. I certainly have drifted my 5e game into the direction of simulation quite a bit.
I use gritty rests and healer's kit dependency, so it easier to treat any HP loss as some sort of actual injury (albeit how severe might be a tad vague) and I also interpret high level characters to be like mythic heroes or wuxia characters rather than real world people, so them being able to do ludicrous stuff and survive implausible things is at least party diegetic in the setting.
I also treat classes as somewhat diegetic and have NPCs (loosely) follow the PC class structures. For example arcane tricksters are known as the Spell Thieves of Shimbal, a tradition with its own history and place in the setting. This also helps in part to avoid "the issekai effect," as by choosing the basic building blocks of your character you are not just choosing arbitrary power packages, but actually something that tells us about the characters place in the world. Also spell levels, known as "eight circles of magic" are part of the lore (eight, because ninth is so rare that its existence is not generally known.)
But most "sim" part of D&D is easily the skill system. And that is exactly the sort of rules-light broad-strokes simulationism I like. There is a numerical bonus that measures the character's capability in the skill and there is a DC that measures how challenging the task is. Both are diegetic. Then from these we draw the odds of succeeding at the task. This is the level of simulation I need. I absolutely do not need seven thousand Skillmaster charts to tediously roll on.
But from here I segue into another matter that was under discussion, narrativism, fail forward, and why some implementations of these might feel unimmersive to many. I think the rune deciphering
@pemerton mentioned is a perfect example of this. The character is just deciphering the runes, not in the fiction, nor in the character's understanding, does this act determine the nature of the runes. However, the player is doing something quite different. They are rolling to see whether they get to decide what the runes are. This makes the player decision space and the character decision space fundamentally disconnected. In the fiction it does not matter which character tries to decipher the runes, except perhaps a less skilled one not being able to do so as effectively. This is the reality of the characters. However to the players at the table it matters a lot. It determines which player gets to input the meaning for the runes, and it means that a less skilled character has higher chance to result the runes being something negative.
It also seems to me rather confused to derive the odds of unrelated phenomenon from things that purportedly measure the character skill and the difficulty of the task. Like why are we drawing odds of the runes being something good from the PC's skill in rune reading? This is not related, hell, obviously the skill in rune reading doesn't measure that at all, as the character will succeed in reading them regardless, it just so happens that people bad at deciphering runes for some reason more often find unfortunate messages as they just as skilfully read those runes!
So should the player play their character as the reality works how the character believes? So for example, if my character who is very curious but not so great at rune-reading happens to get to the runes first, should I play them as eagerly trying to read the runes as from their perspective there is no harm in that, even though I as a player know this not to be the case?
Similar sort of disconnect can happen also with some forms of fail forward. I certainly am quite often aware of this tension whilst playing Blades in the Dark. It is no wonder that this is something a lot of people find unimmersive, as it literally forces a disconnect between the player and the character. And to be fair, there are also things in D&D and games like it that can cause somewhat similar disconnect. Oddities caused by gamifying the combat structure is common source of this, and as a result, I tend to find combats the least immersive part of D&D play experience.
But of course none of this means that these things are bad. Rigid structure of D&D combat makes them more tactical, and a lot of people find that fun. And allowing players to affect the shared fiction in non-causal manner in narrativist games certainly opens up new possibilities to player to add to the story. It is just a question of whether you consider the trade-offs to be worth it.
Also, relating to how GM implementation affects the achieved simulation in D&D, there absolutely is nothing stopping someone from drifting it into more narrativistic direction instead. One could have absolutely ran Pemerton's rune example in D&D as an arcana skill check.