@adammlevy I’ve updated the link, thanks for pointing this out.
These many-years-old links are broken because the file is not anymore on the alpha branch. To access the file, you need to go back in time when the file existed by selecting an old version tag on the GitHub page, then you will see the file. For example, there is a link in the Parse Server 3 migration guide you mentioned that points to the Parse JS SDK 2.0 migration guide, that also does not exist anymore. If you select the version tag 3.0.0 in the Parse JS SDK repository then you can access the file: Parse-SDK-JS/2.0.0.md at 3.0.0 · parse-community/Parse-SDK-JS · GitHub.