How to install the addon:

1. Download the Old Auto Smooth addon as a zip file.

2. In Blender, go to Edit > Preferences > Add-ons.

3. Click the Install button at the top right corner of the window.

4. Navigate to the zip file and select it, then click Install Add-on.

5. Enable the Old Auto Smooth addon by checking the box next to its name in the Add-ons list.

Here's how to use the add-on:

1. After installing the add-on, locate the old 'Normals' panel in the Object Data tab within the Properties area.

2. Here, you can toggle auto smooth ON/OFF and adjust the auto smooth angle. The add-on will manage the auto smooth modifier in the background automatically.

3. Alternatively, you can right-click in the viewport and select "Shade Smooth by Angle (OAS)" to achieve the same effect, akin to using Shade Smooth or Shade Flat.