In Instana, when an agent goes offline or stops reporting, this is reflected as a “Not Reporting” status in the UI, and an event is generated indicating that the host is no longer reporting telemetry (often surfaced as a host/agent availability issue). Instana’s Agents page shows both Reporting and Not Reporting agents, so you can see disconnected agents directly in the UI without any alerting setup.
This page explains how to view or manage host agents: https://www.ibm.com/docs/en/instana-observability?topic=agents-managing-host
However, notifications are not sent automatically by default unless you configure an alert based on that event. If your goal is to be notified when an agent disconnects, an approach can be to create an alert using Host Availability Detection (or the corresponding host availability event) and route it to your preferred alert channel (such as email, Slack, PagerDuty, or other integrations).
This page explains how to define a custom event in Instana: Defining Custom Events In Instana