So, the removable orthodontic appliance is for kids that can wear and remove. Usually, they are done to correct any sort of movement identified earlier.
So, instead of having full-on braces, let's say if you have one tooth that is too far in, we ask you to wear this for maybe a couple of months to get the effect.
It is actually similar to Invisalign, so yes it is still in use. Though it might not be as popular, it highlights the importance that you can get yourself checked early on before puberty if possible because this is the phase where we can correct a lot of problems very easily on a child.
If you wait till you are an adult, it will be a much different kind of treatment. We are talking about extraction, surgery, and longer use of braces.