It seems not to work to throttle stage{} or steps{} blocks  in a declarative Jenkinsfile.

When throttle is used outside of a steps block I get an syntax error.
When I use throttle inside a steps block it doesn't have any effect.

I have a throttle category named build configured to run max. 4 jobs per Node, max. total concurrent builds is 0.
It works when I'm using throttle() in the scripted syntax.

With the following job the throttling category has no effect:

pipeline {
agent any

  stages {
    stage("sleep") {
      steps {
        throttle(["build"]) {
          sh 'echo sleeping; sleep 120;'
        }
      }
    }
  }
}

When using a node inside throttle, throttling works but the declarative syntax requires a label parameter for node().
That has the disadvantages:

• It's not possible to run the step on any node and throttle it, except you tag all jenkins machines with the same label.
• a new node uses a different workspace
• an executor is consumed for the node block, if it's throttled the executor will be only waiting for being allowed to run

steps {
  throttle(["build"]) {
    node(label: "master") {
      sh 'echo sleeping; sleep 120;'
    }
  }
}

(I also couldn't find any documentation about how to use it in declarative syntax)