A masking functionality has been requested by the community for many years – both here in Community and on GitHub.
Sadly, it seems to be pretty low priority. At first glance, this doesn’t seem particularly hard to implement – it’s essentially an “alpha” blending method, simple math. But it looks like Aseprite’s method for storing and displaying color is kinda messy under the hood (documented here), so who knows. Maybe this isn’t currently doable at all.
Anyway, I’d love to hear @dacap’s word on this – has there been any progress on this topic?