Workloads that either are superseded or there is a desire to supersede them. It's not just old deprecated stuff, but also stuff that people want to deprecate.

An example could be old API endpoints only hit by old versions of a mobile app where users may be slow to update. Or a case where only newer clients/instances support modern features and old ones are stuck on a frozen featureset until they cutover.

But active services with no immediately existant alternative may also be legacy. If your auth is behind a disappointing managed service like AWS Cognito, you may consider your entire auth stack legacy while the replacement is still looming in the roadmap down the line. Maybe you can't justify funding to work on a transition just yet, but you already would avoid building on top of the existing tech stack.