I was curious about this claim, so I searched for oneof in the googleapis repo[0][1]. There's a bunch of results, all in proto3, and many were committed for the first time (at least publicly) in the last 4-6 months. So I don't believe this is true.

From a safety perspective, tagged union types, which oneofs are, are incredibly useful and really should be used more often.

[0]: https://github.com/googleapis/googleapis/search?q=oneof&unsc...

[1]: I have no idea what this repo actually does, to be honest, but it appears authoritative about something, and appears to be public grpc apis Google exposes.