Are we living in middle ages that electronic records don't exist of what happened in the past engagement. Maybe one can locate the last McKinsey dossier on OneDrive? The problem is and always was of incentives of people involved.
Only time that answer is different is when a person in power is still around and they pretty much veto the engagement with a consulting company.
Their reputation doesn’t come from the quality of their work but their size: they can just muscle their way in.
It is also quite difficult to surface useful information amongst all the noise in the giant archive of documents