diff --git a/wagtail/admin/modal_workflow.py b/wagtail/admin/modal_workflow.py
index 119cc0e61f..75fd98b70b 100644
--- a/wagtail/admin/modal_workflow.py
+++ b/wagtail/admin/modal_workflow.py
@@ -1,49 +1,22 @@
-import json
-import warnings
-
-from django.http import HttpResponse, JsonResponse
+from django.http import JsonResponse
 from django.template.loader import render_to_string
 
-from wagtail.utils.deprecation import RemovedInWagtail24Warning
 
-
-def render_modal_workflow(request, html_template, js_template, template_vars=None, json_data=None):
+def render_modal_workflow(request, html_template, js_template=None, template_vars=None, json_data=None):
     """"
     Render a response consisting of an HTML chunk and a JS onload chunk
     in the format required by the modal-workflow framework.
     """
     if js_template:
-        warnings.warn(
-            "Passing a JS template to render_modal_workflow is deprecated. "
-            "Use an 'onload' dict on the ModalWorkflow constructor instead",
-            category=RemovedInWagtail24Warning
-        )
-        # construct response as Javascript, including a JS function as the 'onload' field
-        response_keyvars = []
+        raise TypeError("Passing a js_template argument to render_modal_workflow is no longer supported")
 
-        if html_template:
-            html = render_to_string(html_template, template_vars or {}, request=request)
-            response_keyvars.append('"html": %s' % json.dumps(html))
+    # construct response as JSON
+    response = {}
 
-        js = render_to_string(js_template, template_vars or {}, request=request)
-        response_keyvars.append('"onload": %s' % js)
+    if html_template:
+        response['html'] = render_to_string(html_template, template_vars or {}, request=request)
 
-        if json_data:
-            for key, val in json_data.items():
-                response_keyvars.append("%s: %s" % (json.dumps(key), json.dumps(val)))
+    if json_data:
+        response.update(json_data)
 
-        response_text = "{%s}" % ','.join(response_keyvars)
-
-        return HttpResponse(response_text, content_type="text/javascript")
-
-    else:
-        # construct response as JSON
-        response = {}
-
-        if html_template:
-            response['html'] = render_to_string(html_template, template_vars or {}, request=request)
-
-        if json_data:
-            response.update(json_data)
-
-        return JsonResponse(response)
+    return JsonResponse(response)
diff --git a/wagtail/admin/static_src/wagtailadmin/js/modal-workflow.js b/wagtail/admin/static_src/wagtailadmin/js/modal-workflow.js
index cd988ecaa9..a470cf7565 100644
--- a/wagtail/admin/static_src/wagtailadmin/js/modal-workflow.js
+++ b/wagtail/admin/static_src/wagtailadmin/js/modal-workflow.js
@@ -55,15 +55,7 @@ function ModalWorkflow(opts) {
     };
 
     self.loadResponseText = function(responseText, textStatus, xhr) {
-        /* RemovedInWagtail24Warning - support for eval()-ing text/javascript responses
-        (rather than JSON.parse) will be dropped */
-        var response;
-        if (xhr && xhr.getResponseHeader('content-type') != 'text/javascript') {
-            response = JSON.parse(responseText);
-        } else {
-            response = eval('(' + responseText + ')');
-        }
-
+        var response = JSON.parse(responseText);
         self.loadBody(response);
     };
 
@@ -74,12 +66,6 @@ function ModalWorkflow(opts) {
             container.modal('show');
         }
 
-        if (response.onload) {
-            // if response contains an 'onload' function, call it
-            // (passing this modal object and the full response data)
-            response.onload(self, response);
-        }
-
         /* If response contains a 'step' identifier, and that identifier is found in
         the onload dict, call that onload handler */
         if (opts.onload && response.step && (response.step in opts.onload)) {