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https://store.steampowered.com/app/2687970/TRIBES_3_Rivals/
You can request Access to join the alpha playtest until Dec 8 evening New York, USA time.
Game has or is planning to implement EasyAntiCheat based on the splashscreen. The alpha test has CtF 5on5 and CtF 12on12.
Game runs on my Ubuntu machine without graphical glitches or unusual stuttering. Streaming it to my friends over Discord caused a major fps drop.
I didn't have to do anything special to get it to run after downloading. It was a seamless experience running it.
Didn't have any Problems playing (the one round that I did play). If one has played Tribes before one will feel right at home. Pretty sure they switched the default keybindings up a bit. Shift for sliding and Space for the jetpack.