use jenkin’s ExtendedChoiceParameterDefinition for multi-select user input

        def userInput = input(
            id: 'userInput',
            message: 'Promote:',
            parameters: [
                [
                    name: 'valuesToMultiSelectFrom',
                    $class: 'ExtendedChoiceParameterDefinition',
                    type: 'PT_MULTI_SELECT',
                    value: 'value1,value2',
                    description: 'Select a value'
                ],
                [
                    name: 'additionalValuesAsCheckboxes',
                    $class: 'ExtendedChoiceParameterDefinition',
                    type: 'PT_CHECKBOX',
                    value: 'additional-value1,additional-value2',
                    description: 'Select additional values'
                ],
            ]
        )

        println(userInput);

        def String[] values = userInput['valuesToMultiSelectFrom'].split(',');
        def (additionalValue1, additionalValue2) = userInput['additionalValuesAsCheckboxes'].tokenize(',');

Check Jenkins’ own documentation for an unnecessarily complicated but at least almost complete overview of available input declarations: https://jenkins.io/doc/pipeline/steps/pipeline-input-step/

Get a list of available types here: https://github.com/jenkinsci/extended-choice-parameter-plugin/blob/master/src/main/java/com/cwctravel/hudson/plugins/extended_choice_parameter/ExtendedChoiceParameterDefinition.java#L90

Get a list of all parameters of the ExtendedChoiceParamterDefinition here: https://github.com/jenkinsci/extended-choice-parameter-plugin/blob/master/src/main/java/com/cwctravel/hudson/plugins/extended_choice_parameter/ExtendedChoiceParameterDefinition.java#L344

This SO was useful to get an idea of how to use it in pipelines and resulted in above example: https://stackoverflow.com/questions/42392247/how-can-i-use-the-extended-choice-parameter-plugin-in-a-jenkins-pipeline-script
This was also somewhat helpful: https://support.cloudbees.com/hc/en-us/articles/115003895271-How-to-do-a-multiselect-input-in-a-pipeline