I'm skeptical that YouTube would be using this for anything remotely significant given Google's strong propensity to only use internal technologies that they fully control.
This was opensourced by google founders were @ google and wrote the thing
They swithed to spanner.
I doubt YouTube is using the cloud service. If they're not 100% on Google's services by now, they still use Vitess. Vitess is the open source project PlanetScale manages which lets you horizontally scale MySQL databases.