Is it a bad thing that Hades doesn't have an official Linux client if it runs perfectly under Proton? They seem to be willing to invest heavily to get the game onto more platforms considering that (AFAIK) they rewrote the entire game in C++ to be able to port it to Switch, so it would surprise me if this is a "we don't care about you" situation and not a "Proton is good enough and doing a from-scratch Linux port would be expensive" situation. For their games before Hades they were using a portable middleware layer (FNA or MonoGame) and the C++-based engine they licensed for Hades is not the same.
I definitely do not believe that Supergiant's choice here was motivated in any way by Proton. I think the costs simply didn't justify a custom port for them this time if making it a better experience would be difficult.
P.S. Hades has an official Vulkan client, not just Direct3D, so the gap between native and emulated on Proton is much smaller than it would be for many games.