FOSS FPS "Phantom Overload" was updated to version 0.3 recently. This update adds a new world type, a new phantom appearence, new settings, footstep sounds, and more. The code is licensed using GPLv3. The game was made using the FOSS Godot engine. It uses the Vulkan API. You can either build the game from source or download releases from Codeberg.
The main website for the project is here:
https://huitsi.net/#PhantomOverload
The source code is available on Codeberg:
https://codeberg.org/Huitsi/PhantomOverload
The developer has a presence on Mastodon: