tl;dr
Tags that represent branches and environments are being replaced with first class support for branches, environments, deployments and releases.
"Tags" in the Pact Broker are simple string values that belong to application version resources (aka. "pacticipant versions") that are generally used to track the git branches and environments associated with an application version. They allow us to identify specific versions when testing backwards compatability and introducing new features, and when determining if an application is safe to deploy. Tags have the advantage of being flexible and generic enough to support any development workflow.
Tags have their disadvantages though. They are similar enough to Docker tags that users expect them to work the same way, but different enough that pages of documentation need to be written to explain how to use them. The main problem with tags though, is that no semantic information that can be inferred from them - that is, it's impossible for the Broker to know whether a tag represents a git branch, an environment, or something else entirely.