And the answer is… the GUI of the Jenkins Github plugin is broken, it never saves what you type into the Override Hook URL box. It has been broken since September 2020, see Jira JENKINS-60738.
The workaround is to manually edit the Jenkins github-plugin-configuration.xml
configuration file to add a field of this form:
<hookUrl>https://smee.io/thestringfromsmee</hookUrl>
…[this example for my case, using SMEE] inside <github-plugin-configuration>
, at the same level as <configs>
and <hookSecretConfigs>
, then restart Jenkins.
Ho hum.