Oh, that helps to jog my memory. Here was the post that started it all, Steem Dollar Stability Enhancements, and here are the HF14 release notes. And, yeah, it's there in the release notes, sort-of...

When Steem Dollars reach 10% of the market cap, the price feed will jump off the peg to keep Steem Dollars at 10% of the market cap and will return to the peg when the reported price feed goes below 10%. Witnesses should still report the real dollar value of Steem for their price feed. The median price feed is automatically capped regardless of what witnesses report. (emphasis added)

And now I see where this came from in the comments of the first post, Steem Dollars have Limits.

There are two extreme cases, black swans, that must be considered. In one extreme the price goes to 0, in the other extreme the price goes to infinity. It is in the interest of everyone involved that the value of a Steem Dollar never hits either extreme. This can be implemented in objective terms by setting hard limits on the exchange rate range that is allowed.

We can set a maximum and minimum Steem Dollar price as follows:

maximum price = SteemSupply / (10 * SteemDollarSupply)
minimum price = SteemSupply / (10000 * SteemDollarSupply)

With these rules in place, Steem Dollar holders are protected against US dollar hyperinflation and Steem holders are protected against US dollar “hyperdeflation”. The witnesses are trusted to publish a honest price feed relative to dollars at all times, but the blockchain will enforce these hard limits.

In other words, the market cap of all Steem Dollars will always be between 0.01% and 10% of the Steem market cap.

Not sure, but my guess is that those percentages were just someone's hunch for reasonable values to use. I wonder if they also implemented that 0.01% rule. I don't see that in the release notes, and I don't remember hearing about it, so maybe not.

P.S. That discussion is painful. Herding cats. ; -)