Since the first PC release of State of Decay 2 in May 2018, there have been a few ways to deal with the need to adjust FOV to preference or to compensate for the decreased vertical FOV at 21:9 and wider resolutions. Some - like the trainers based on Cheat Engine - were effective but are no longer up to date, while others - like the YFOV command - would not allow for any fine-tuning. There's a better way.

We'll be assuming the game has been launched at least once to create a config file.

• Navigate to %LOCALAPPDATA%\StateOfDecay2\Saved\Config\WindowsNoEditor\
  (it can be pasted into the address bar of the file explorer)
  
  
• Open Engine.ini in a text editor
  
  
• Add the following lines:
  
  [SystemSettings]
  r.FieldOfView=95
  
  The provided value is arbitrary but it is also the value needed to compensate for the decreased FOV at a 21:9 resolution to match the default 80 of 16:9.
  For 32:9, go for 118.
  For 48:9, set it to 137.
  Needless to say, it can be set to any reasonable horizontal value without breaking the game!

The FOV-related commands previously shared by others are the known go-to commands for all games built on Unreal Engine 4 but have severe limitations.

• [/script/engine.localplayer]
  AspectRatioAxisConstraint=AspectRatio_MaintainYFOV
  
  This one is the most common in conversations and guides on State of Decay 2.
  There are also hilariously random modifications like AspectRatioAxisConstraint=90
  The game will read it as AspectRatioAxisConstraint=, defaulting to the first option within its index.
  Any of them, including AspectRatioAxisConstraint=IdontKnowWhatImDoing will work as a result.
  
  The only thing they do is set the hardcoded FOV of 80 to be vertical rather than horizontal, producing an FOV of 112 at 16:9 or roughly 126 at 21:9 - arguably too much with no way to adjust it.
  
  
  
• [/script/daytongame.daytongameusersettings]
  FOV=90
  
  FOV is a native Unreal Engine 4 command but there is no documented evidence of it working via the config in State of Decay 2.

After a user request on the WSGF subreddit, I installed the Game Pass version of the game just to look into the issue. Having a history of modding games for ultrawide support[community.pcgamingwiki.com], I started by disassembling the game and tracing its code for FOV operations. The r.FieldOfView string is something I almost accidentally stumbled upon when looking into making a tool for real-time FOV adjustment.
I was quick to share the command on WSGF and also publish it on PCGamingWiki.