Being constantly contactable is pretty new, and yet we're already at the point where being left-on-read can be weirdly stressful. Before that, maybe a message on an answering machine, no idea when/if it would be heard, and the risk of not being home when/if a call would be returned.
The red light on the answer phone has been replaced with a red dot on an app icon. Maybe with a big number. And maybe on many apps. And always with some expectation of acknowledgement.