DRM-free your thing? Shop on GOG regularly? Good news, Motion Twin/Evil Empire have now sorted the DLC situation for Linux on GOG with Dead Cells.
Now even more people can enjoy the awesome looking and brilliant combat in Dead Cells, with the expanded content in the recent Dead Cells: The Bad Seed DLC which is absolutely worth picking up. It's helped me personally enjoy the game for quite a few more hours as it nicely mixes up with early game and the extras are excellent.
While some developers are able to easily get DLC up, it can get a little complicated with DRM-free builds at times. It does depend on a number of factors, in this case it's GOG missing some API functions for their Linux builds that caused the hold-up. Speaking on Reddit, a rep for the developer said this after I queried it:
No DRM, but depending on platform, the way they "install" dlc data on the user system vary. So they supply some API to allow us to interface with their technical restriction and that also ensure compatibility over time with their store (in case they do change that part). Gog is surprinsingly [sic] do not provide that on Linux. We did implement it in a "hack" manner, so that is working now (or should be very soon enabled on their store). But not related to any kind of DRM stuff.