When updating to the latest Bitbucket Branch Source version 937.0.1 (or 937.0.0) using CasC no longer allows injection the server URL with an environment variable. I double checked by manually adding a configuration and then viewing the updated CasC in Jenkins. Via that I renamed "serverUrl" to "serverURL", but it didn't fix the issue.

My updated YAML:

unclassified:
  bitbucketEndpointConfiguration:
    endpoints:
      - bitbucketServerEndpoint:
          displayName: "Bitbucket"
          serverURL: "${JENKINS_BITBUCKET_SERVER_URL}"

For some reason CasC doesn't replace the serverURL with the environment variable, in the System configuration I see this now:

When I downgrade back to 936.4.4 and use my old YAML it works fine:

unclassified:
  bitbucketEndpointConfiguration:
    endpoints:
      - bitbucketServerEndpoint:
          displayName: "Bitbucket"
          serverUrl: "${JENKINS_BITBUCKET_SERVER_URL}"

The server URL gets the correct value as set by the environment variable JENKINS_BITBUCKET_SERVER_URL: