All upstream Bazel ebuilds install a binary called bazel and so are all in SLOT 0 as multiple versions cannot co-exist. For Chrome OS, we want the ability to carry multiple LTS versions of Bazel for long-term compatibility with the various individual packages that use Bazel. These dev-util/bazel ebuilds differ from upstream in that they install a binary suffixed with the major version of the release (e.g. bazel-5) and have their SLOT set accordingly (e.g. SLOT=5). This means that multiple LTS releases can be installed simultaneously, but only one of each major version.
This package will remain forked unless upstream were to adopt a similar SLOT scheme along with a way for ebuilds dependent on Bazel to declare which versions of Bazel they are compatible with.