At-least as far back as 2017 when I wrote Schillsaver: https://github.com/Valkryst/Schillsaver

None of us, in the original discussion threads, knew of it being done before then IIRC.

The idea of exploiting someone else's server to store files is incredibly old.

https://en.wikipedia.org/wiki/GMail_Drive

When Google launched Gmail (2004) with a huge 1GB storage quota, Richard Jones released GMailFS to mount a Gmail account as a standard block device.

I mean, it is pretty likely they figured out it could be a pretty obvious possible misuse before anyone actually started doing it.