Which? Apart from the twitter case, which was ages ago and largely overblown, there's no case that I can remember of ditching ruby for tech X because "for scale". There are lots of examples of companies using other tech rather than ruby, but usually for different reasons (python ML tooling, go for some performance bottleneck, Java for hiring).
"I worked at Twitch from 2014 to 2018... No more Ruby on Rails because no good way was found to scale it organizationally; almost everything is now Go"
Ruby gets results.