Currently, we can capture the output of a shell step inside of a script block, but have no way to do that in clean declarative pipeline syntax. For example, this post on StackOverflow must instead be:

// Git committer email
script {
  GIT_COMMIT_EMAIL = sh (
    script: 'git --no-pager show -s --format=\'%ae\'',
    returnStdout: true
  ).trim()
}
echo "Git committer email: ${GIT_COMMIT_EMAIL}"

It also doesn't work to wrap it in a declarative environment block, though that may perhaps be a reasonable place to extend the syntax.