Summary:
Given a Declarative Pipeline with a boolean parameter, which has a default setting of true, one might use this boolean to conditionally execute, or not execute, a particular stage. The syntax I've used to do this is similar to that shown in JENKINS-44298. It appears that changing this boolean at runtime, via the Jenkins UI, has no effect on whether or not the stage gets run.

Assumptions:
I'm working under the assumption that, a boolean parameter set up in the parameters

} condition written like this:

            when {
                expression {
                    return TRUE_OR_FALSE
                }
            }

is a valid way of deciding whether or not a stage gets run. Here, if TRUE_OR_FALSE is set to `true`, it'll run. If it's set to false, it won't. Similarly, if it were written like this:

            when {
                expression {
                    return !TRUE_OR_FALSE
                }
            }

The stage only gets run if TRUE_OR_FALSE is set to `false`.

I found this approach while working on issue JENKINS-44298.

Steps to recreate:
1. Create a new Pipeline job on your Jenkins, and use this as its code:

pipeline {
    agent any
    parameters {
        // Default value here is true.
        booleanParam(name: 'TRUE_OR_FALSE', defaultValue: true, description: 'This boolean defaults to true!') 
    }
    stages {
        stage('parallel-1') {
            when {
                expression {
                    // Given our default value is true, this should 
                    // run if I don't change the parameter from its 
                    // default value of true, to false.
                    return TRUE_OR_FALSE
                }
            }
            steps {
                echo "MUST BE TRUE"
            } // end of steps
        } // end of stage
    } // end stages
}

2. Run this Pipeline by clicking "Build" on the left. The first time you do so, it'll fail anyway, because of a different bug that I'll look for / open separately. Nevermind that for now.

3. Do a full page refresh in the UI, and the "Build" link has changed to "Build with parameters." Click this link, and leave the defaults. The boolean will be defaulted to True:

Go ahead and run it this way, and it'll run correctly.

4. Now, run it again, this time changing the parameter to false:

5. The stage will still run, as if you had not set the parameter to false.