Zulip has been powered by Django since the very early days of its development with Django 1.4, back in 2012. As a reasonably mature web application with significant scale, we're at the stage in many companies' development where one starts to rip out more and more of the web framework to optimize things or just make them work the way we want. (E.g. while I was at Dropbox in early 2016, we discovered we only had about 600 lines of code left from the original Pylons framework that actually ran).
One of the things that has been really fantastic about Django is that we're still happily using it for the vast majority of code in the project, and every time Django comes out with a new release, I read the changelog and get excited about several improvements that actually make my life better. While Django has made some design decisions that I don't agree with (e.g. I'm not a fan of Django REST framework, and think it makes life more difficult), Django also makes it easy to do your own thing, which we've done to great effect (see the linked article for details on our
Overall I think we've gotten a ton of value out of Python and Django and would recommend it to anyone starting a new full-featured web application project today.