In a sibling comment I noted wayland referencing some kernel values[0]; and I'd hope there would be some reasonable standard. I certainly didn't have to manually configure chrome & firefox for them to know which buttons on my mouse to map to previous/next page. Maybe the more exotic mouses would need some, but I'd still expect in such case that the user is able to say "ok this button is 'back'" to their DE, and not necessarily be forced to configure that for every single application individually.
That said, I do think it would be nice to have written down that 8/9 are commonly used as backwards/forwards. They are just not explicitly backward/forward buttons though.
[0]: https://github.com/torvalds/linux/blob/4a0fc73da97efd23a383c...