I'm not sure if I understood the original feature, but the general idea would be:

1. Be able to associate a mask layer to a regular layer (or group layer)
2. The mask layer should be editable like a grayscale/background layer (8-bit), grayscale levels from 0 to 255.
3. As a special case, the mask layer might be the composition of several other layers (i.e. the mask layer might be a group layer)

In the community: https://community.aseprite.org/t/layer-mask-for-color-and-transparency/311

+1 for this

Another +1 from me, this would be dope. The alpha lock doesn't always cut it.

Mask layer real is a top notch feature that I use in Photoshop/Procreate, would really like it here

Really need this - it's very frustrating having to use either the selection tool or just paint precisely in order to do non-destructive shadows or highlights on a separate layer. Not to mention, having to edit all layers of the group when you want to make a small change.

Just want to throw my hat in the ring as well, recently I was working on a piece where I wanted to use masking to paint in things only to find out its not a feature.

want to do this: i need to mask something where the layer i work on only appears on one layer beneath it, but its super irritating to not have it as a feature, especially when trying layer effects to see how it looks on ONLY the ONE layer beneath it

Maybe 2022 will be the year we can have this incredibly useful feature? Pretty please?

+1, first git comment as well.
Having the ability to clip a layer to another layer's opacity is so powerful, its the only thing I'm really missing from photoshop other than layer styles. I utilize aseprite with blender and other plugins and having that power all in one place would be really fantastic.