DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
Was introduced in the time that GitLab still used NFS, which is not required anymore in most cases. By removing this, the API it calls will return empty responses. This interface has to be removed in the next major release, expected to be 12.0.