A few weeks ago we - a small group at Seeq, my employer - announced the pr-reminders bot open source project.

The pr-reminders bot is the bot that we have been using to send timely reminders to our teammates about pull requests awaiting review. We have found it to be helpful across multiple teams to keep the code flowing to production. And now you can set it up for your team. See the announcement and/or the repository for more details - including how to deploy it yourself and how to contribute.