AMD FidelityFX Super Resolution (FSR), a performance improvement graphical setting, was added to the game with the Warrior Priest update, Patch 4.5.0, but you need to enable TAA for it to work. If TAA is disabled in the settings, then you can’t enable FSR.
FXAA has a lower performance hit than TAA. And since the reason FSR is enabled is to improve performance, it makes sense to allow it to work with FXAA, rather than make it exclusive to TAA which has a higher performance hit. (I think FSR looks better with FXAA than TAA too.)
However, there is a workaround to make FSR work with FXAA: if you enable FXAA in the game settings, close the game, then edit the game’s config file, (user_settings.config) and set fsr_enabled = true from fsr_enabled = false, then the next time you start the game, FSR will work with FXAA instead of TAA.
A downside to this workaround is that any further changes to the video settings will disable FSR and you’ll have to exit the game, edit the config file and start the game again. I’m making this post as a request to allow users to set FSR with FXAA in the game settings so that we won’t have to keep editing config files and restarting the game if we change video settings.
Back 4 Blood, Black Desert Online, Dota 2, and probably many other games that also support FSR, allow you to use FSR with FXAA.