Deployment is part of dev, so no, we don't typically use --omit=dev or --only=prod in the deployment pipeline.

It is usual (and actually pretty important) to have --omit=dev or --only=prod in the test pipeline.

This often means we do need separate build environments in CICD for test and for deployment.

Specifically, what is important is that some part of the CICD process will fail if a production dependency is missing. Normally, that's the unit test run, supported by a --omit=dev or --only=prod flag to be sure that no upstream dev dependency causes a false success.

This outcome can be achieved in various other ways, but is usually achieved by separating the test and deployment environments.

Edit: to cloud matters further, there's plenty of cases where not bothering with --omit=dev or --only=prod during test run is fine. So you will still see healthy well run projects that do not separate test from deploy, and that's not necessarily a mistake.

But the best practice is a best practice for good reasons, so naturally we stick with it when unsure.