I'd be interested to know what were the decisions that led to switching back. Was the content creation process hindered by Markdown + git? Were there growing pains that couldn't be solved?
Yes, it is. We made the decision to move over (...again) to WordPress last year. This is hosted externally.
As you guessed, I asked that we move this over because the content creation process wasn't just hindered, it was broken.
Non-technical authors couldn't contribute without a developer assisting and pushing it live. This became an impossible situation as we started to scale up our content and diversity of authors.
Making Jekyll function the way we wanted would have required significant development work, and maintaining it or building out new functions would have drained resources from other products we need to focus on at Stack Overflow.
Moving this into WordPress and hosting it elsewhere not only made the content team's life much easier, it freed up our engineers from having to reply to every "Hey, I have a blog post..." request.
We're all a lot happier for it.