I have Photoshop documents with many layers, some over 100.
I need to delete a feathered selection on all layers without having to manually select each layer, reapply the selection, hit Delete key.
Is this possible?
Once you have your marquee selection, it automatically applies to any layer you select.
You can simply keep selecting each layer (Use Alt+[/] to move down/up layers) and just hit Delete (it'll go pretty quickly)
Notice the lines getting removed from each layer.
You can even turn it into an action and add a hotkey to do it for you (If you have many documents it'll be helpful - just make the selection, hit the hotkey and wallah-- it's done)
(You will have to do it fully once to create the action)
If it's just 1 doc with 100 layers, record yourself doing it to a few layers and then just hit the hotkey a few times.
My answer is to simplify your file first, and then apply the feathering. Creating groups or file duplicates helps^. after duplicating them (the Group and/or the image) you can merge grouped sections and apply special filters, etc. (in this case feathering) to the merged group.
The groups and duplicates can also help you restore any portion of the file you may not be able to go back in your history to reclaim.
(^Layers / select layers [keep them together] Cntrl G.) & (Image - Duplicate)
You may begin to form groups naturally as you work. They make multi tasking on multiple layers, like your example, easier.