I mean: has anyone captured every shiny that can possibly exist?
As far as I know, the answer is a definite resounding no. There are plenty of people with every pokemon shiny and they obviously hacked.
I have seen some collections people have that are almost complete but they are comprised of shinnies self caught as well as traded and wonder traded shinnies. It is impossible to verify the legitmateness of traded and wonder traded shinnies. There are people who literally upload hacked shinnies all day on wonder trade.
The closest I seen in a collection comprised of only self caught shinnies, they were in the 200 range of different shinnies (not counting same species like say multiple shiny magikarp caught while fishing but only counting one of them for dex purposes). However I also take this with a grain of salt because some of them hack in items such as masterballs or tickets to get access to event pokemon, so this for me puts their entire collection into question. Plus I don't think it is fair to count shinnies that they normally wouldn't have had access too without hacking.
Me personally I have 70 something different shinnies and most of those were caught back in the 8k odds days. I plan on starting up again and going for a complete shiny dex. But to put how arduous this will be into perspective, it would take a minimum of two years of catching a new shiny every single day to complete it (with all legitimate possible shinnies that is). Sure you can get lucky and catch multiple shinnies in one day, I think my record is 4. But there will also be days you catch nothing. Not to mention you would need multiple games across multiple generations to get every single pokemon shiny legimate. Some these are only accessible in the 8k odds games.
That is not to say that it is impossible, only that realistically speaking it would take enormous dedication, luck and time. Anyone who claims otherwise is either lying, hacking or naive.
Just for final thought, Sun and Moon are bringing new pokemon which means even more shinnies to try and catch. So be prepared to be in the hunting business for the next couple years if you truly want a complete legit shiny dex.