GameObject Brush Replaces Prefab Brush
How to use the Prefab Brush in Unity 2020+
// or not.
Ok, before you get your hopes up the prefab brush is gone. As of 2D TileMap Extras 1.6 or later and Unity 2020 or later you can no longer use the Prefab Brush in Unity Tile Palettes. However all is not lost. While the Prefab Brush has been removed a replacement exists, kinda.
Starting in 2D TileMap Extras 1.6 and newer you now have to use the GameObject Brush to get most of the prefab brush functionality.