I’ve been looking some on the net, and reading countless posts and documentation in forms of Wikipedia, stackexchange and on ext4.wiki.kernel.org. But I’m trying to find the root documentation that mentions exactly what happens what it goes to 100%.
I’ve heard that it should become read only on 100%, but I’d like the actual creditable source for it.
To my understanding this should be Distro/OS independent
I dont see any reason why you should not try it yourself in a VM or a spare computer (put a small partition then fill it).
Is this just personal curiousity or are you writing a white paper of sorts?
Could you guys do it and post here please?
I’m also curious.
Internal documentation in the likes of Wikipedia, where we actually source where we find the information before we publish it 
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Don’t take this as official documentation, but i have experienced out of space on ext4 in production.
The system was read only IIRC until we expanded it.
However, this is the big one - it isn’t at 100%, as there is some space reserved for the root user. You will hit this issue prior to 100% utilisation (iirc, root-reserved space is set at filesystem creation, and also IIRC it defaults to 5% reserved for root).
This should be fairly simple for you to evaluate though - build a VM (or even a loopback filesystem) and fill it up 
The same sort of thing happens if you run out of inodes (you should be concerned with that too, as some dumb apps create heaps of zero byte files). I’ve seen that as well.
Easy, if it doesn’t exist, document your experiment and cite that.
You could reach out to the the FileSystem maintainer and get it from the horses mouth.
I have experience with this and it pretty much happens as @thro stated. But if you are looking for documentation, reach out to the people that maintain it. If there is no official documentation, you may just be able to get your self an open source contribution credit by working with them and publishing the documentation.
I have done this quite a few times with ext4 and basically writes error out like if the drive was readonly, but you can still delete files to free up space and once you do everything starts up again.