Working with VS Code, the explorer git status gutter indicators stopped showing for one of my repositories.
Those didn't resolve the issue.
(Git changes are still tracked in the Source Control Manager panel)
Apparently another common reason for this is that you are accessing your git repo through a symlink. See: https://github.com/microsoft/vscode/issues/60010#issuecomment-433424527
This was the case for me and redirecting vscode to open the folder via the full path fixed it.