As Fabien said in this comment:

I think that the [bundle] extension system should never be used for anything but Open-Source/shared bundles.

But if you read the article about creating an extension class you don't see a big warning message saying: "DON'T USE THIS in your Symfony app. This is only useful if you create bundles published as third-party bundles."

I feel that the same goes for other articles, but I cannot share here specific links to those articles.

So, I'd like to revisit all the articles related to bundles, simplify the articles related to in-app bundles and better separate them from articles related to third-party bundles.