I didn't disappear on purpose, I just work 12 hour shifts. Let me see if I can vaguely comment on some things.
The thread isn't about playing characters with severe cognitive impairments. If that is what you're doing, well you know what you're doing. (side note too - Those people are usually a lot 'smarter' than people give them credit for. They often get infantilized and treated as less than human and that is something we should be challenging within ourselves as well).
The thread is about how the concept of 'intelligence' as understood colloquially is a bunch of nonsense. Yes "IQ" and "G Intelligence" too.
The concept we have of intelligence in our culture is so ingrained that people take it for granted. A truism.
Multiple people have have referred to intelligence as inherent, going so far as to say it's something a person is born with. This is dangerous thinking which has been used to support eugenics (not saying that is what people meant by it when posting).
It has been pointed out that Intelligence originally in D&D was thought to correlate to the IQ scale. The IQ scale is a load of nonsense and so is D&D's original concept of intelligence. Thankfully in 5e intelligence is very narrow. Part of the point of the OP is to not draw broad conclusions of the character because of what the trait of 'intelligence' is called but to look instead at what it actually does in game.
Many cognitive traits and abilities our culture values we label as intelligence and then we label those who are lacking in those traits as 'stupid'.
Even if we were to grant that what our culture values is inherently good and right and we call some of that intelligence it still isn't correct to say it is inherent. There are countless factors both internal and external that are going to change, sometimes drastically, how intelligent a person is perceived to be.
A lot of "intelligence" is actually either a measurement of accumulated knowledge or the result of behaviours. This is why I listed a few traits and behaviours in the OP. Some people responded 'but that isn't what intelligence is.' And that is the point. Those traits and behaviours can make someone appear to us as unintelligent. Someone made reference to ADHD and said something along the lines of that doesn't make someone 'stupid' they just have traits and behaviours which make it difficult to succeed in our culture which expects different things from them. Many people with ADHD have reported that they thought of themselves as 'stupid' before their diagnosis. That's the problem, that's the harm.
There are numerous others. Find something someone is internally motivated to learn and they're going to have a much easier time learning it. Teach things in different ways - esp. different than in books - and a lot of people will do better. People who can learn from books tend to have a lot more accumulated knowledge because that is what we have valued. Now thankfully with new technologies many people have access to different ways to learn. Sometimes it's a matter of addressing cognitive distortions which inhibit learning. These are often learned and reinforced and can be addressed. It might be a matter of teaching someone from a different culture what the culture they're in values and how to learn and adapt to it. Maybe it's a matter of addressing 'learned helplessness' where the person doesn't apply themselves or attempt to accumulate knowledge because they have a negative self image wherein they believe they are incapable of doing so. Etc. Etc.
I didn't create this thread because something catastrophic happened. This is a result of seeing hundreds of threads about 'how to play PC or NPC with X intelligence' and other such things and seeing what people view as 'smart' and 'stupid'. I'm just challenging the common framing, that's all.
It's not about being 'offended' by what someone does in their basement. It's about how they think of other people and how they reinforce their beliefs. The roleplaying as an event in their basement isn't harmful. It's what they do after. It's not about directly treating people poorly (but please don't), it's about reinforcing harmful beliefs and attitudes which hurt others.
Our culture views some people as 'lesser' and we should fight that wherever we can.
So, don't play a 'stupid' character but do give a character traits and behaviours that get in the way.
Please don't take this post as being exhaustive on the topic. This was a rambling post too where I'm responding to various ideas in this thread, there is no central thesis that is being argued coherently throughout. It's not a research paper.