Hi Patrick,
Thank you for your question. Yes, undeleting testcases is possible. So far we do not support it over the App, but with some dev-wizardry it is possible.
Just open the Dev-Console and undelete it by providing the test case id.
await testiny.testcase.undelete({id:5})
To find the ID, best take a look at the Auditlog (https://app.testiny.io/settings/audit-log).