I heard that YouTube has migrated to Spanner already. YT was the last major user of MySQL at Google, the Googlers probably consider that as a snowflake and wanted to move away from it. Nothing wrong with Vitess itself.
Vitess was most definitely a special snowflake: most Google production servers don't even have disk anymore[0].
(There are a whole host of other reasons for doing Vitess->Spanner, all of which fall under "special snowflake infrastructure".)
[0] https://static.googleusercontent.com/media/sre.google/en//st... (see ch2, diskless)