Is the API used by IBM's internal hooks for GitLab public or private?

We are currently using V6.0.5 of RTC.  We have a team that has developed hooks to link GitLab merge requests with RTC work items.  Their scripts are based on the API used by IBM's internal hooks for Gitlab.  The team wanted to add more feedback to the user when errors occurred.  Is the API used by the internal IBM hooks public or private?  We upgrade RTC 1-2 times a year and want to understand the potential work involved maintaining these scripts.  

Thanks,

Jim