I started working on cross-signing in @Matrix.org with the initial spec proposal https://github.com/matrix-org/matrix-doc/pull/1681 submitted in September 2018. Over a year and a half later, it's nearly ready to go live in Riot.im. We had a video-conferenced key-signing party today at work, which involved us yelling emoji over each other. It's nice to have people stress-testing something that you've worked on for so long, and have it hold up.