IPHONEOS_DEPLOYMENT_TARGET is set to 8.0, but the range of supported deployment target versions is 9.0 to 14.0.99

Code Block 
post_install do |installer|
  installer.pods_project.targets.each do |target|
    target.build_configurations.each do |config|
      if Gem::Version.new('8.0') > Gem::Version.new(config.build_settings['IPHONEOS_DEPLOYMENT_TARGET'])
        config.build_settings['IPHONEOS_DEPLOYMENT_TARGET'] = '8.0'
      end
    end
  end
end

reference

Is this reply, by utyyyreee, a workaround or a fix?

I have the same issue, although for me it does not pose a problem as it is only affecting tests and those warnings don't actually result in errors down the line (i.e. the code is actually not relying on anything iOS 8.0 specific).

In short: You are correct in your assumption that the problem lies with a dependency further down the line. That means you cannot do much yourself, I believe, as the swift package manager has no means of manipulating that dependency and its settings. In cocoapods you can do that with a snippet similar to the one that utyyyreee posted (without any context... and a typo in lines 4 and 5, where it should say 9.0 and not 8.0... and that only if you want to set it arbitrarily to 9.0...).

That would go into your podfile to manage the project's pod dependencies. Sadly, since it happens in a dependency you yourself only include via the swift package manager, you cannot do it yourself but have to approach the owner of that dependency to make the necessary changes. Or you wait until the underlying dependencies get updated accordingly on their own.

See also this article here for an explanation (no idea why the forum won't let me properly link it, sorry):

Code Block 
https://www.jessesquires.com/blog/2020/07/20/xcode-12-drops-support-for-ios-8-fix-for-cocoapods/

Lastly, I assume the pod thing won't help you at all, as the swift package manager probably circumvents that completely (unless they do some weird shenanigans).

After all, the code is not included via pods, but packages.

I, for example, use Telegraph and that simply has the .iOS(.v8) platform defined in its Package.swift file. I will probably contact the author and/or fork the repo to get that to be updated. I don't know whether that then also incrementally updates any dependencies further down the line or whether they also have to be updated in their respective Package.swift files...

You can set the Deployment target for all your Pods references :

Code Block 
post_install do |installer|
 installer.pods_project.targets.each do |target|
  target.build_configurations.each do |config|
   config.build_settings['IPHONEOS_DEPLOYMENT_TARGET'] = '9.0'
  end
 end
end

Or remove the value:

Code Block 
post_install do |installer|
 installer.pods_project.targets.each do |target|
  target.build_configurations.each do |config|
   config.build_settings.delete 'IPHONEOS_DEPLOYMENT_TARGET'
  end
 end
end