Clara might be a bit more popular than Followup.cc. We know about 1 link to it since March 2021 and only 1 link to Followup.cc. We are tracking product recommendations and mentions on various public social media platforms and blogs. They can help you identify which product is more popular and what people think of it.
I'll update this if I find more. For now, this seems to be a partial solution. It's a Gmail add on (except I don't use Gmail) that adds reminders. The options when setting reminders include cancelling it if someone replies, and to remind everyone in the thread. If you combine those two, it seems like — if you use Gmail's web client and have $23/month you're dying to get rid of — a decent way to add the feature. Source: over 1 year ago
Seems like I'm about 5 years late to the party, because Clara labs does exactly this; https://claralabs.com/. Source: about 1 year ago
FollowUpThen - The simplest way to schedule an email reminder
x.ai - x.ai is a tool to schedule meetings for you and your team.
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Julie Desk - Virtual assistant using AI to schedule your meetings
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Mastermind Manager - Software for running mastermind groups