Bitbucket Server 5.0 has deprecated the legacy hook API, quoting from the API Changelog:
New Hooks API
In Bitbucket Server 5.0 we are introducting a new API for Hooks. This API will replace the existing hooks API and the merge checks API. This new API makes retrieving added and removed commits much simpler and more efficient. For more information on the new API and how to use it, see the Repository Hooks and Merge Checks Guide.
The legacy Hooks API has been deprecated for removal in Bitbucket Server 6.0.
Bitbucket Server 5.0 has deprecated the legacy hook API, quoting from the API Changelog: