If you do embedded development, things like https://platformio.org/platformio-ide, but also smaller, nice to have extensions for auto-deploying code to cloud providers, etc.
To me that sounds like claiming Arduino IDE is a "crucial extension". Their website[1] lists a bunch of IDEs where it can be integrated, so I wouldn't call it missing. That said both of these are hobbyist toys to make it more approachable and embedded development was fine long before VSCode, they're in no way "crucial".