Facebook-owned WhatsApp has begun sending cease and desist letters out to developers whose apps interact with, interfere with or use WhatsApp and its APIs, even if the apps only do so through the use of native APIs present in the core Android OS. WhatsApp's terms of service state quite clearly that developers cannot make offshoots, clients, or other types of apps that directly hook into WhatsApp's services and divert users from the main WhatsApp official client, but now the app's legal team is even going after developers whose apps display information from WhatsApp directly, such as through the use of Android's native notification APIs.
One such case is found linked in the Reddit post in the source link, pertaining to called DirectChat. The app hooks into Android's native notification API, and pops up all your notifications in the form of a bubble that draws over other apps and does not interfere with them, much like the Chat Heads popularized by Facebook Messenger. The app does not store any conversations or dismiss notifications by default, but the developer still got a letter from WhatsApp. Many other developers with similar apps have received similar letters, which stipulate that the app makers agreed to the terms of service for WhatsApp in order to use WhatsApp's data and services, and that limited license is revoked, which means that developers who get these letters are effectively banned from WhatsApp, even for personal use.
The legal argument against these letters, in most cases, is a simple one; the developers in question are not using WhatsApp APIs, or storing any data that comes directly from WhatsApp. In using the Android notification listeners, many apps, such as Tasker, may display data from an app on a secondhand basis, without actually interacting with the app at all. From a legal standpoint, this would, in theory, make those apps immune to the terms of service of apps that they access through the Android notifications API, including WhatsApp. Many of these developers do not have the resources to go into a legal battle with WhatsApp, and are looking at potentially banding together on that front. It is worth noting that WhatsApp, or any other party for that matter, can send a cease and desist without merit, which may be what's happened here. For the time being, WhatsApp and Facebook have shown no signs of serious intent to bring these developers to court, but that may happen in the near future. The letters only started rolling in recently, and they all say that the developers have seven days to respond.