This works on my local (having Apache2) and on my Compute Engine having NGINX.
My views.py :

from django.shortcuts import render
from django.http import StreamingHttpResponse
import time

def home(request):
    return render(request, 'home.html', {})

def automate_run(request):
    # content_type doesn't have to be application/json
    response = StreamingHttpResponse(automate_foo(), content_type="application/json")
    response['Cache-Control'] = 'no-cache'  # prevent client cache
    response["X-Accel-Buffering"] = "no"  # Allow Stream over NGINX server
    return response

def automate_foo():
    for i in range(1, 11):
        status = "Step %d" % i
        percentage = i * 10
        json_string = f'{{"status": "{status}","percentage": "{percentage}"}}'
        print(json_string)
        yield json_string
        time.sleep(.5)

But response["X-Accel-Buffering"] = "no" doesn’t seem to have any effect on Google App Engine which I understand uses NGINX under the hood.

Demo : https://cloudxchange.el.r.appspot.com (right-click > Inspect)