Here is some advice from Google's John Mueller on Reddit. He said, "my go to answer for these kind of questions is if it's not documented it could change over time." The question was on how Google treats 301 versus 308 redirects.
I mean, the advice sounds good and all - but even things that are documented can and does change. I guess things that are not documented probably can change more frequently or it shows Google does not want to put anything in writing.
But Google has changed their documentation many many times when they want to change things on their end.
But nevertheless, interesting line from Google - don't you think?
Forum discussion at Reddit.