I recently wondered how long it had been since I added ethitter.com to the Strict Transport Security preload list, as it’s been several years. I added my domain without fully understanding the consequences, though I’ve been fortunate to avoid any problems that could’ve resulted.
Getting back to my original question, version control made it easy to find August 18, 2014 as the date my domain landed in Chromium’s list: https://src.chromium.org/viewvc/chrome?view=revision&revision=290306. At the time, it was one of less than 1,000 domains in the preload list; the bulk of the list was comprised of Google’s own domains. There are currently over 40,000 preloaded domains in Chrome/Chromium. It’s a good thing preloading hasn’t been an issue, it takes a while to get off the list.