A few years ago we reported that Google may treat a 308 redirects as a 301 redirect. Then shortly later we said you can't rely on that because it was not documented. Three years later it was added to the official Google documents that Google treats 308 redirects like 301 redirects - so now it is official.
Now if you look at the revised docs, it says they are the same, Google wrote "the 301 and 308 status codes mean that a page has permanently moved to a new location."
Gary Illyes from Google also confirmed this change recently on Twitter:
we just merge that with 301 so we really don't care iirc
— Gary 鯨理／경리 Illyes (@methode) July 9, 2021
Forum discussion at Twitter.
