where do we draw the line between accommodating for the fact that all people generally are different in the way they think and act and allowing it to be used as an excuse for them to get away with behaving terribly towards other people

The thing I always said is it's a reason not an excuse. Someone might be an arsehole because of whatever but that doesn't mean they're absolved from apologising for their behaviour (if they realise later or it's pointed out to them) and doing better (and not making excuses about why it's the other person's problem but a lot of people do that).

These days everyone seems to want to class themselves as being ADHD or on the autism spectrum

I think your follow on sentence about finally having answers for at the time inexplicable behaviour patterns/straight up not understanding some things/etc is very much a thing. In some cases I feel that people just claim such because they view other people diagnosed (professionally or self) getting "special" treatment and want in on that, or they're looking for a free pass to be a dick (I feel like everyone has had that friend or at least seen either in meatspace or online that one person or even several people who are obnoxious and when called out cry about it being their autism/adhd and they can't be held accountable and you're ableist if you do).

My theory is that in western culture we are awkward not because of neurodivergence, but because we've become so far removed from our community dynamics

Hard agree. We do the rock up at houses of friends/family that we're close with like we own the place too and jump right in to help, and we also have to explain it to new friends to the house if they're not used to it or used to people who are not used to it.

_{I tend to comment as I'm reading and then I got partway down and saw you drew the same conclusions XD}

Random thoughts going on random journeys are the most entertaining (or the most time consuming sometimes).

I also skimed the comments and saw one that mentioned chatting over a fence, can't do that much anymore they're too damn high. One of our neighbours called over the fence the other day because she needed help with something and she had to stand on a tree stump or something that was on her side to look over XD