June 2017. See
https://steemit.com/steemit/@thecryptodrive/welcome-to-steem-hf-19
And this:
https://steemit.com/steemvoter/@steemvoter/steemvoter-week-in-review-for-may-21-26
Perhaps the most notable change to your voting well-being is that your 100% vote will post-hardfork be 4 times as strong as before the hard-fork, but will also drain 4 times more power. So if you previously were voting 40 posts per day at 100% you can now only vote 10 posts per day at 100% without draining your vote power any lower than what you have been used to.
It was combined with a linear voting system instead of the previous exponential one.
I think they thought they were taking power away from the whales and giving it to minnows. But what actually happened is that the small group of power voters who were keeping the site stable by dishing out 40 votes a day to anything that caught their eye were discouraged and everyone else just self-voted and you get the mess we see now.
Yes, it seems that backfired big time. There might not have foresee that eleven month ago as I suspect minnow and plankton votes where not as week and wales not as powerful.