There used to be an option to set a channel called, "Mix" as a capture device in KMix, which I used a long time ago to record sound outputs rather than(/as well as?) sound inputs. Has this been removed from KMix, ALSA or somewhere else sound-related? (P.S. I've uninstalled Pulseaudio)

If you cannot see this channel is "alsamixer" then it was removed by ALSA. If it is present in ALSA mixer, then right click on empty space in the KMix window, and select Channels.

Hmm. It's not in Alsamixer either. I did check in the Channel Selection dialogue first, then in alsa mixer. Sucks. :/