How to undo split normals? Is there a simple way to undo split normals? It seems that it can be done with Python code like described in Clear Custom Split Normals Data on all objects but how to do this for one object using the normal Blender user interface?
Steps to reproduce the problem I'm having (Blender version 2.90.1):

Create a new Blender file.
Delete default cube.
Add new mesh: Monkey
Right click (or any other way to open the Object context menu) the Monkey (Suzanne) and select &quot;Shade smooth&quot;.
In Properties view: Object data properties, check Auto smooth and set angle to 50.

The top of the Monkey is now smooth but ears still have sharp edges. Adjusting the Auto Smooth angle results in live update to smoothing of the Monkey.

While Monkey is selected press TAB to enter Edit Mode
Select Mesh - Normals - Split from 3D Viewport menu.

Auto smooth is now broken. How to undo the split normals so that Auto Smooth angle setting of Monkey still works as between the steps 5 and 6? (Obviously I'm not asking how to press Ctrl+Z, imagine that there has been lots of edits to scene after the step 7.)
It seems that at least Mesh - Normals - Merge is not the correct answer. I would have expected merge to be opposite of split but this doesn't seem to be true here.
@scurest had the correct answer for this one. With Blender version 2.92 it seems undoing Mesh - Normals - Split is a bit harder:

Select the object and click &quot;Clear Custom Split Normals Data&quot; in properties editor - Object Data Properties - Geometry Data. (See image below.)
Press TAB to enter Object Edit mode (or select it from viewport top left corner).
Press key a to select all (or use Select - All from menu).
Select Edge - Clear Sharp from the viewport top menu.

It seems that with Blender version 2.92 the action Mesh - Normals - Split does both adding custom split normals data and marking all affected edges as sharp. To undo that, both must be fixed.
This is documented at https://docs.blender.org/manual/en/2.92/modeling/meshes/structure.html#modeling-meshes-normals-custom