Here are the two ways that I lock items in Google Slides. Google doesn't offer a way to do this, so I demonstrate two hacks. Well, they are hacks in a good way, like how I lightly toast my bread for PB&J sandwiches to make it easier to spread the peanut butter.
One of the ways is to save the Google Slides as an image. You can then take this image and set it as the background for another slide. Since all of the objects are parts of the same image, no one can modify the objects.
The other way is to use the Master slides feature. In Google Slides, you are not limited to the built in layouts. You can create custom layouts for your slides. These custom layouts lock the objects to the slide. The only way to modify them is to edit the master slide.
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