The reason i haven't developed code for this is that i have never been acknowledged by any programmers at steem inc. So, since this would be code that would change a great deal the load and processing done by the witnesses,... i would never even start this journey without the go ahead from someone with the authority to implement it.
I am pretty sure the algorithm just starts at the last block, and then goes backwards through the blocks until it gets 20 and then returns the list. Which means anything that is not current runs through a lot of blocks.
There is that, too! But to be fair, there are many thousands of users and they have no real way to communicate with all of them or to know who they are.. I think the steemdev discord rooms might be the best way in to communicate - or github.. However, I rarely get replies from anyone at steemit on anything anyway.
There are 22 results combing back in the 'created' list for the tag I created for my witness posts - so I'm not so sure there's a numerical limit:
https://steempeak.com/created/urasoul-witness