My Cloud SQL instance is currently on 9.6.6, I just got an email from GCP saying they are upgrading to 9.6.11 next week. There is a data corruption bug mentioned in the 9.6.7 release notes, not sure if it's the one the author is referring to.

https://www.postgresql.org/docs/9.6/release-9-6-7.html

"Fix vacuuming of tuples that were updated while key-share locked (Andres Freund, Álvaro Herrera)

In some cases VACUUM would fail to remove such tuples even though they are now dead, leading to assorted data corruption scenarios."