With the latest update to the Minecraft launcher, there is an advertisement for Realms day from December 17th. This advertisement is annoying because it shifts the usual position of the "Play" button upwards and puts a "Read more" button for the ad in its place. This, on its own, is manageable, but the fact that it doesn't disappear after clicking "Read more", combined with the lack of a button to remove it, makes the launcher annoying to use.
Hello! This is actually a news feed integrated into the main window, which was introduced in this version. If you scroll down, you'll see the other news posts! The fact that this can't be removed is very likely not a bug, but if you'd like to give feedback, you can do so here.