Branch write protection plugin (1.3) is installed together with path write protection plugin (1.1), but only the latter one is actually available.
Both mentioned plugins were installed on SCM manager instance directly connected to the internet, and then whole plugins folder is transferred to the instance which is offline.
According to the logs, both plugins were loaded:
2019-05-13 13:26:46.386 [main] INFO sonia.scm.plugin.DefaultPluginLoader - load plugin /net/nfs/vol010/sn00810373/.scm/plugins/sonia/scm/plugins/scm-pathwp-plugin/1.1/scm-pathwp-plugin-1.1.jar
2019-05-13 13:26:46.388 [main] INFO sonia.scm.plugin.DefaultPluginLoader - load plugin /net/nfs/vol010/sn00810373/.scm/plugins/sonia/scm/plugins/scm-branchwp-plugin/1.3/scm-branchwp-plugin-1.3.jar
Screenshots and complete logs are attached.
SCM manager version is: 1.6.
OS version is: Red Hat Enterprise Linux Server release 7.5 (Maipo)
#1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64 x86_64 x86_64 GNU/Linux
Java version is: OpenJDK 64-Bit Server VM 18.9 (build 11.0.3+7-LTS, mixed mode, sharing)
Let me know if you need additional information.