Major versions of the BrAPI spec are accompanied by broad changes to the whole specification. Major versions are not backward compatible with each other. When a new major version is built, all the deprecated fields are permanently removed, allowing for a clean slate to start again. This is why it is important to indicate the major version in the URL for every BrAPI call. If a server needs to support multiple major versions of the spec, each version will be implemented under a different base path.
Minor versions are typically smaller updates to the spec, fixing a group of smaller github issues like new search parameters, additional data fields, and mistakes not following best practice. Minor versions are backward compatible, so a single response can have all the data fields necessary to support the needs of multiple minor versions.
Deprecated fields, parameters, and endpoints indicate that something has been removed or replaced in a minor version update.