On the topic of slideouts, I see that many use a retractable awning click here for photo, which I assume is designed to keep rain water from entering inside the cabin space.

Is this the best method or only method that is in use?

Is there anything that is replacing this system or something that is better?

Hi Dog,
I don't know of, or can't think of, anything other than a slide awning that would serve the purpose. Obviously, a side benefit of the awning - it also provides some shade. They are virtually maintenance free however, it does make it difficult to dust off the top of the slide. If you park in a dusty area (such as in the desert) it's a good idea to knock the dust off before you retract the slide.

If you were in a dusty environment and wanted to get the dust off the top of the slide out so when you retract the slideout, the dust doesn't enter the cabin space, can you unhook the awning, and then go up there with a brush or air hose and brush or blow the dust off?

Or is the awning device sort of permanently attached making that idea difficult?

Do all slideouts use the awning device or are some manufacturers leaving them off...?

As the awning fabric is attached to a rail (which is screwed to the wall of the RV, it would be impractical to disconnect. Also, the roller is fastened to the slide under spring tension.
Some manufacturers offer the slide awning as an option, some as standard equipment.