One of the changes in Blender 2.8 is that Windows can now be split from any corner. Because of that the indicators from previous versions were removed instead of adding them to every corner.
The problem that I encounter too often though is that a few buttons are too small and to close to these window corners. This makes it hard to click them because they are in some cases, like the "Add new Collection" button in the outliner, inaccessable by a third of their size.
One of the solutions that I can see is to add a bit of extra distance to the buttons from the corners so that users don't accidentally split the windows all the time.
This is a bit harder to do when the window is too slim to show the entire row of buttons, like in a single collumn properties editor.
The icons that are just barely displayed are basically inaccessible anyway since they are blocked by the split window corner. I don't know a good solution to that.
But my suggestion would be to add the indicators back and to every corner, BUT they are only overlayed if the mouse moves close to a corner, letting the user know if buttons are partially blocked at the moment and where exactly in the corner the window can be split.
This overlay can be similar to how a scroll bar appears if a window is not long enough to display all of its contents and the mouse hovers over the side of the window.