Comments such as this one read to me as a dig at a particular technology the commenter does not like. Sort of a more subtle version of "Oh, I didn't realize people were still using THAT."

Yes, I think even more so now with the Node Foundation and support for all modern JS syntax (ES6 and beyond). It's being used by mayor players (LinkeIn, Netflix, Medium and Uber to name a few) but also smaller companies (from my personal experience as a freelance developer).

PS Assuming that by web app you mean "the backend for web apps". Otherwise Node would be used 'just' for the build process, together with Webpack and friends.

It's still a popular choice. It's very easy to get a simple API or service up and running in production with node, especially if you don't try and include every new library out there for node projects. Express, Rails, and Flask/Django aren't going anywhere anytime soon since they're accessible frameworks in popular scripting languages.

What gave you that impression and where exactly do you get your information?

On the contrary, I would say its popularity is increasing more and more every day.

My company is almost exclusively using Node for our backends.

What would have replaced it?