There is a bugzilla entry that states the removal was due to too little user benefit for the development effort required. And since I don’t necessarily need this feature, I can understand they directed the resources to where they are needed more. Nevertheless, it would be nice to have.

The way I use it is primarily for applications that produce audio output, so I get appropriately named per-app volume sliders in pulseaudio/pipewire and not just a bunch of audio streams titled “Firefox”.