How do I lock a layer in Blender 2.8?

I want to lock my reference plane. https://i.imgur.com/KGFuphk.jpg

You can make a collection/object unselectable (the arrow icon next to it in the layout)

Do you mean this part?

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No, there’s an icon you have to enable in 2.8 (was visible by default in 2.79) order to disability the ability to select an object/collection:

After you turn that icon on, clicking it will keep you from being able to select that object in the viewport:

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