IdentityServer offers multiple diagnostics possibilites. The logs contains detailed information and are your best friend when troubleshooting. For security reasons the error messages returned to the UI/client are very brief - the logs always have all the details of what went wrong.
Open Telemetry is the new standard way of emiting diagnostics information from a process and IdentityServer supports Traces (.NET Activities), Metrics and Logs.
The eventing system was created as an extension point to integrate with application monitoring systems (APM). They used to have their own different APIs so IdentityServer only provided events that could be used to call the APM’s APIs. Thanks to Open Telemetry there is now a standardized way to emit diagnostic information from a process. The events may eventually be deprecated and removed.