I get following error:
message "Cannot cast object 'com.atlassian.plugin.servlet.PluginHttpRequestWrapper@631bb09c' with class 'com.sun.proxy.$Proxy2885' to class 'groovy.lang.Closure'"
stack-trace "org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'com.atlassian.plugin.servlet.PluginHttpRequestWrapper@631bb09c' with class 'com.sun.proxy.$Proxy2885' to class 'groovy.lang.Closure'\n\tat com.onresolve.scriptrunner.runner.rest.common.CustomEndpointDelegate.methodMissing(CustomEndpointDelegate.groovy:20)\n\tat Script559$_run_closure1.doCall(Script559.groovy:30)\n\tat com.onresolve.scriptrunner.runner.rest.common.UserCustomScriptEndpoint.doEndpoint(UserCustomScriptEndpoint.groovy:308)\n\tat com.onresolve.scriptrunner.runner.rest.common.UserCustomScriptEndpoint.getUserEndpoint(UserCustomScriptEndpoint.groovy:193)\n"
status-code "INTERNAL_SERVER_ERROR"

With this script:

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.search.SearchProvider
import com.atlassian.jira.jql.parser.JqlQueryParser
import com.atlassian.jira.web.bean.PagerFilter
import com.atlassian.jira.user.util.DefaultUserManager
import com.atlassian.crowd.embedded.api.User
import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.jira.issue.Issue
import com.atlassian.jira.ComponentManager
import java.text.SimpleDateFormat
import java.util.Date

import com.atlassian.jira.component.ComponentAccessor
import com.atlassian.sal.api.user.UserManager
import com.onresolve.scriptrunner.runner.rest.common.CustomEndpointDelegate
import groovy.json.JsonBuilder
import groovy.transform.BaseScript

import javax.servlet.http.HttpServletRequest
import javax.ws.rs.core.Response
import javax.ws.rs.core.MultivaluedMap

@BaseScript CustomEndpointDelegate delegate


retrieveReleaseNotes(
    httpMethod: "GET", groups: ["jira-administrators"]
) { MultivaluedMap queryParams, String body, HttpServletRequest request ->

def extraPath2 = getAdditionalPath(request)
def extraPath = "test"

def jqlQueryParser = ComponentAccessor.getComponent(JqlQueryParser)
def searchProvider = ComponentAccessor.getComponent(SearchProvider)
def issueManager = ComponentAccessor.getIssueManager()
def userManager = ComponentAccessor.getUserManager();

// Adjust HERE the buildnumber to a certain version, where you want to retrieve all closed devs
def buildNumber = 49

def user = ComponentAccessor.getJiraAuthenticationContext().loggedInUser
def query = jqlQueryParser.parseQuery('project = DIV AND issuetype = Development AND status = Closed AND fixVersion = 2.8 AND "Build Number" ~ ' + buildNumber.toString())
def results = searchProvider.search(query, user, PagerFilter.getUnlimitedFilter())
def closedDevs = "";
def closedDevList = [];
def featureList = [];
def bugList = [];
results.getIssues().each {issue ->
    Issue parentIssue = issue.parentObject;
    def issueName = parentIssue.getIssueType().name.toUpperCase();
    def issueSummary = issue.getSummary();
    if (issueSummary.substring(0, 8) == "[DEV] - ")
        issueSummary = issueSummary.substring(8, issueSummary.length());

    closedDevs += "* {{$issueName}} " + issueSummary + " {{JIRA|$issue.key}}<br>";

    def text = "* {{$issueName}} " + issueSummary + " {{JIRA|$issue.key}}";
    if (issueName == "FEATURE")
        featureList << text;
    else
        bugList << text;
}

def metadata = "=== Version 2.8.0 - Build $buildNumber ===<br>";
    metadata += "<br>";
    metadata += "Release: " + new Date().format('YYYY-MM-dd') + "<br><br>";

featureList.each {
    metadata += "${it}<br>";
}

bugList.each {
    metadata += "${it}<br>";
}


    return Response.ok(new JsonBuilder([releaseNotes: metadata, extra: extraPath]).toString()).build();
}

The error seems to be because of

def extraPath2 = getAdditionalPath(request)