To name a few: AssaultCube, Battle for Wesnoth, Cube2: Sauerbraten, FligthGear, Freeciv, Freeciv21, Nexuiz Classic, OpenArena, OpenHV, OpenRA, OpenTTD, Remnants of the Precursors, SpeeDreams, Stone Kingdom, SuperTux, SuperTuxKart, Unciv, Urban Terror, Veloren, Warozone 2100, Widelands, Xonotic
P.S. It may be that not all of them are FOSS, but they run natively on linux.
You could use an RSS reader app and add a .rss to the end of a Mastodon URL. It works for individual accounts, hashtags, and instances.