Despite the previous report that it likely wouldn't work on Desktop Linux, Blue Protocol: Star Resonance is out now and does work. Now you too can enjoy the anime gacha on your preferred platform be it Desktop Linux or Steam Deck with SteamOS.
It's supposedly using Anti-Cheat Expert (ACE), but it's not actually listed on the Steam page, which as per Valve's rules it's supposed to be. Currently this is a little unclear on what exactly it's using since there's no proper statement now from the developer. Either way, it works.
However, there are some issues so it's not completely playable yet.
A bunch of the videos don't play by default in Proton, and you just get a black screen. I've tried Proton 9, Experimental and the latest GE-Proton and they all have the same issue. They're somewhat minor with the videos for picking a class and area introductions but it still detracts from the experience. Aside from that, everything else does appear to work and quite nicely too.

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If you do like these free to play anime styled MMOs, then at least it's another choice for you on Linux right now. From what they said it has a "mega-server", so there's only one server that everyone is one in a shared world.
Highlights from the press email:
- Deep Character Customization: Players become the stars of their own anime with an extensive creation suite featuring 127 adjustable parameters and a global dye system covering over 250 parts — allowing them to shape virtually any anime-inspired character they can imagine.. They design every detail of their unique hero, from their appearance and fashion to their evolving combat styles and builds.
- A Vast Open World to Explore: Players will journey through the stunning, vibrant landscapes of Regnus. They discover hidden dungeons, scale majestic peaks, and uncover the secrets of a world teeming with life and mystery.
- Thrilling & Dynamic Combat: Players choose from eight distinct classes, each with a deeply unique playstyle and ability set, from swift melee strikers to powerful ranged spellcasters. They team up to unleash spectacular combos and take down massive, screen-filling bosses, each featuring unique mechanics and phases that require strategy and teamwork to overcome.
- Your Community Awaits: Connect with friends through guilds, housing, and diverse social events. In Regnus, every playstyle is celebrated. Whether you live for combat, master life skills, or just enjoy the mini-games, you will find your home here. The greatest adventures are shared!

I'm not sure how they're trying to make it work. From what I see in the steam reviews, many users are complaining about the high ping.
Source [External Link]