It looks like, right now, you can access 1.9 documentation on the website, like so: https://www.yoctoproject.org/docs/1.9/toaster-manual/toaster-manual.html Those 1.9 document pages should probably redirect to the 2.0 equivalents: https://www.yoctoproject.org/docs/2.0/toaster-manual/toaster-manual.html
I added the redirect to a page with a short explanation. Edits are welcome.