VRR works fine in Proton and on Plasma's Wayland compositor. I don't think the game supports VRR, as I saw reports of windows users lamenting that the game doesn't support it either.
This is my experience too. However, there is nothing special a game needs to do to support VRR. So the fact that VRR works fine in this game under Windows but not Linux makes me think there is a bug in Proton, the compositor, or the GPU driver.
I can say with 100% certainty that VRR is working as expected under Windows 11 with my RTX 3080. I haven’t tested in Fedora yet.
Hate to jump in 4-5 days after you posting, but what are your Steam launch options?
I see on ProtonDB a lot of people are having to fidget with some Vulkan settings, and I myself can't even get the game to launch as I get the "Graphics card does not meet minimum requirement" message when I have a RX 6700XT. Granted I'm not running the game through Steam and I'm trying to play via Lutris with wine-ge. If you're wondering how I'm "not running the game through Steam", let's just say the waves were rough last night and creating a lot of torrents.
One thing I tried was editing the current build of Windows to be 11 and not 10, but that doesn't seem to work. Looking at the ProtonDB page, there's these following environment variables people here can try adding to their launch options (though don't add all of them):
ENABLE_VKBASALT=1
AMD_VULKAN_ICD=RADV
PROTON_HIDE_NVIDIA_GPU=0 PROTON_ENABLE_NAVPI=1 # note the user who supplied this said they had to downgrade their NVIDIA drivers to 525.x
VKD3D_CONFIG=force_compute_root_parameters_push_ubo # the user who supplied this said nothing worked for them
DISABLE_LAYER_AMD_SWITCHABLE_GRAPHICS_1=1 VK_ICD_FILENAMES=/usr/share/vulkan/icd.d/radeon_icd.i686.json:/usr/share/vulkan/icd.d/radeon_icd.x86_64.json # if you have an AMD GPU
DXVK_CONFIG_PATH=/home/niko/.local/share/Steam/steamapps/common/Starfield/dxvk.conf
EDIT: It seems like the following options get rid of the "minimum requirements" message, however now I get "your graphics drivers are out of date."
# Launch options if you're running the game in Steam
DXVK_ASYNC=1 WINE_FULLSCREEN_FSR=1 WINE_FULLSCREEN_FSR_STRENGTH=1 AMD_VULKAN_ICD=RADV VK_ICD_FILENAMES=/usr/share/vulkan/icd.d/radeon_icd.x86_64.json gamemoderun %command% --skip-launcher
This is with wine-ge-proton8-14, don't know if GE-Proton8-14 would make a difference.