Today is the day! Streets Of Rage 4 - Mr. X Nightmare is out now expanding the beat 'em up with a bunch of new content, plus there's a free upgrade for all players. Currently, the Steam page doesn't list Linux on the DLC but the big update to the base game did enable it and according to the porter Ethan Lee you should be good to go.
Mr. X Nightmare adds in Estel Aguirre, Max Thunder and Shiva, a new survival mode and leaderboard, new moves and weapons plus new music tracks.
For the free update you can expect to see a new Mania + difficulty level, an in-depth training mode with a tutorial, new colour palettes for characters and a number of other refinements and balance changes.
Another point that they didn't actually mention in the release notes is how it now has FNA3D hooked up fully in the settings, so you can pick between OpenGL and Vulkan directly in the game. Porter Ethan Lee mentioned how "FNA3D Vulkan has been tested a LOT with SoR4 but it's still in the final stages of development, so if you find anything odd please let us know!" so be sure to report any new issues if you play it with Vulkan.