Well it depends what you consider scale. When you look at the high scale deployments like Slack, YouTube, GitHub, Square, etc they are all using Vitess and MySQL.
YouTube doesn’t use just mysql or vitess these days they migrated to other google databases.
Slack, GitHub both are stupidly shardable. I doubt it’s one RDBMS handling every customer, chat room, git repo. And instead they’ve segmented the workload across multiple instances.
Of course they run on multiple instances. It doesn't work for every use case but it works for more than people realize. Too many people vertically scale, or add read replicas which is a ticking time bomb.
