For legacy app/database migrations, what validation checks actually mattered before cutover?

I’m working in a systems/modernization role and trying to think through this from a release engineering / operational readiness angle, not just “get it deployed and hope." For people who have helped move older enterprise systems into cloud or newer infrastructure — especially SQL-heavy apps, IIS/PHP/.NET-era apps, or systems with a lot of stored procedure/business logic — what validation checks actually mattered before cutover? I’m less interested