GitHub account unavailable on Xcode 10.2
This is kind of a tough one to search for but I did find this SO Q&amp;A titled: Xcode source control wrong username on github
. There's an answer there that shows the use of a CLI tool called xcrun to manipulate your Github credentials that are stored in Xcode, for eg:

$ xcrun git config --local user.name 'new_user_name'
$ xcrun git config --local user.email 'new@email.com'


Might help with getting your issue sorted out.

Xcode 10

I was also able to find reports of people having the same issue as you where their source repos (Bitbucket, Github, etc) were greyed out in Xcode. For eg:



This reddit thread titled: Add GitHub account greyed out on Xcode 10, can't fix mentions this process for manually deleting out the creds. This appears to be a issue with Xcode 10 itself.


  Happened to me, this is the only thing that worked (from StackOverflow):
  
  
  Make screenshots of the custom preference settings
  Delete /Applications/Xcode
  Delete all files containing com.apple.dt
  in ~/Library/Caches
  Delete all files containing com.apple.dt
  in ~/Library/Preferences
  Open App Store.app, download and re-install Xcode.
  Create the Apple ID developer account
  
  
  After those steps all account types were enabled and I could create the GitHub account. Finally restore your custom preference settings from the screenshots.


References


Xcode add Github account not possible