`nextflow.preview` flags should not be errors in the release immediately afterwards

[SPPearce]

The new workflow output definitions have been around for a while, able to be used via setting nextflow.preview.output = true.

Now 25.10.0 has released today, all my pipelines where I have been using this suddenly fail.

Instead of failure, for the next major release the flag should be deprecated instead, and then you remove it afterwards.

[bentsherman]

A deprecation warning isn't good enough because using a preview flag when the feature is out of preview is invalid. This is important to prevent ambiguous or unclear source code, especially now that more code is being read and written by AI

You should always consult the migration notes when upgrading to a new Nextflow version. And when you use preview flags, you should assume that the new version won't work out of the box. I cannot stress this enough

We will try to clarify this in the docs #6504

[bentsherman]

The feature flags page in the Nextflow docs now have a clear warning about the risk of breaking changes in subsequent versions