It was a Go packaging best practice before it was a Go modules best practice. What about your users that don’t use dep? These breaking changes affect them too.
If you’re not concerned about those users, then why not make this new version of badger v1.6.0 instead of v2.0.0? Then you can make it a proper v1 Go module, and when you do decide to move to v2 then you can do it the proper Go module way.