Disclosure: I used to work on GCE (and was adjacent to the serial port work).

IIRC, we launched interactive serial port access sometime in late 2014. For example, mbrukman answered a SO question on Jan 2, 2015 with connect-to-serial-port [1]. I don’t recall when we gained fancier IAM controls for it, but we’ve had it forever (and I think getting / view only was there at public launch).

[1] https://stackoverflow.com/questions/27734763/how-do-you-acce...