My company uses Github for repo storage. I generally check out work repos to ~/Workspace. Other stuff, like my ~/dotfiles are going to other directories. So, here, I use my company wide id_ed25519 key for ssh access to github if I'm in '*/Workspace'. Otherwise, I use credentials scoped to my personal Github account.

.sshconfig

Host  *
  PermitLocalCommand  yes
  StrictHostKeyChecking ask

Host github.com
  HostName github.com
  User git
  IdentitiesOnly yes

Match Host github.com exec "pwd | grep '/Workspace' > /dev/null"
  IdentityFile ~/.ssh/id_ed25519

Match Host github.com !exec "pwd | grep '/Workspace' > /dev/null"
  IdentityFile ~/.ssh/gh_hefeweizen