Add Shadow and Hightlight Adjustment Layer Feature in Photoshop

The problem here is that Shadows/Highlights was more or less rendered obsolete when the Camera Raw filter (ACR filter) was introduced a little later. Its shadow and highlight recovery function is much more effective than the old Shadows/Highlights code, and with fewer artifacts.

The ACR filter borrows a part of the ACR raw processing engine, for use with RGB files inside Photoshop. It should not be confused with the full ACR plugin.

True, the ACR filter is not available as an adjustment layer either. But if one of these two ever will be, chances are it will be the ACR filter. Of course, it can be used as a smart filter already, for basically the same functionality as an adjustment layer, but that requires working with smart objects..

Functions and commands that are strictly superseded by newer functions, generally aren’t removed, so Shadows/Highlights is still around. But I would strongly recommend you give the ACR filter a try. 

ACR filter as adjustment layer is an active feature request.

This would still be a really great feature. Please reconsider. 

I have added my vote, however, it has always been my understanding that the specific image processing used in S/H is not possible in a simple adjustment layer.

That's a very good idea.

In the meantime, if you don't already know, you can use what's known as "Smart Filters" to

have Shadows Highlights act as an adjustment layer of sorts.

With the layer active that you want to adjust with Shadows/Highlights turn the layer to a smart

object by going to Filter>Convert For Smart Filters (or just turn the layer to a Smart Object)

Then after you apply the Image>Adjustments>Shadows/Highlights you can double click on

the Shadows/Highlights in the Layers panel to readjust.

https://helpx.adobe.com/photoshop/using/applying-smart-filters.html