Bitbucket Server 6.x releases are now supported. However, the changes to do so mean the app no longer supports Bitbucket Server 5.0-5.10. The minimum supported version is now 5.11.0.
Instances running Bitbucket Server 5.10 or older must remain on a 1.x release of this app. The major version bump here is to help call out the removal of support for older Bitbucket Server 5.x releases; there are no significant functionality changes in this release.