One of my "Build Flow Plugin" jobs uses an environment variable FUU set by job1. It could be retrieved like:

{{def b = build( "job1")
def fuu = b.environment.get( "FUU" )}}

Note, that job1 is a freestyle (non-pipeline) job.

As Build Flow is deprecated, I am in the process to migrate this job to the pipeline plugin.
I would expect that I can access the environment variables by using a snippet like

{{def b = build 'job1'
def b.buildVariables.FUU}}

This does only return null in this case.

It works correctly if job1 is a pipeline step, which I find counter intuitive.