Elevated 413 or 413 errors could mean that a bug was introduced to the client/frontend that sends large payloads or cookies.

Elevated 400, 401, or 403 errors could mean that a bug was introduced in session or cookie handling middleware, client, or server code.

Elevated 200s means it could be a DDOS attack or issues with client-side polling.

Etc..

Alert on status code anomalies, not on volume/percentage of certain status code.