If you have installed the Configuration as Code plugin, then the "System Configuration" section of the "Manage Jenkins" page includes a "Configuration as Code" link. Click that to navigate to the "Configuration as Code" page. There, click "View Configuration" in the "Actions" section. That generates YAML from the current settings of Jenkins. If you have also installed the Timestamper plugin and set the "Enabled for all Pipeline builds" option, then the displayed YAML includes the allPipelines: true setting that I showed above.
See https://github.com/jenkinsci/timestamper-plugin/pull/63 and JENKINS-62445 for how this was implemented.
Configuration as Code generates the following YAML for that setting:
I have not tested whether adding that to jenkins.yaml changes the setting, but I don't see why it wouldn't.