One imperfect but helpful technique is to use release notes for your application.
Ask the team to write good commits when changing code, and aggregate all code changes done to the application into a page that describes what went in that release. GitHub releases do that for you automatically. When a release is pushed that breaks something, it's easier to scan the release notes and find who changed what.
Another technique is to use explicit feature flags to protect all code changes. Then use a dashboard that shows recent changes to feature flags – again this helps tracking down the culprit.
Ask the team to write good commits when changing code, and aggregate all code changes done to the application into a page that describes what went in that release. GitHub releases do that for you automatically. When a release is pushed that breaks something, it's easier to scan the release notes and find who changed what.
Another technique is to use explicit feature flags to protect all code changes. Then use a dashboard that shows recent changes to feature flags – again this helps tracking down the culprit.