Summary:
Users are not able to delete a sprint once the board has already been deleted. Those sprints are still queryable via JQL.

Steps to Reproduce:

1. Create a Scrum board/project
2. Create a sprint ("Sprint A")
3. Delete the board
4. JQL on "Sprint A" or try to assign "Sprint A" to an issue.

Expected Results:
JQL should not allow to query on orphaned sprints. Sprint field should also not allow to assign issues to an orphan sprint.

Actual Results:
JQL allows to query on the orphaned sprint and the sprint field allows to assign an issue to an orphan sprint.

Workaround

1. Go to the Backlog of any Jira Software Company-Managed (Classic) scrum project.
2. The Backlog contains multiple swimlanes, the first ones are sprints swimlanes, the last one is the backlog swimlane.
3. Select an issue that is not associated with any sprint, an issue that is under the backlog swimlane.
4. Edit the issue sprint field and select the orphan sprint as your target sprint.
5. Now the orphan sprint is visible on the actual board, it should be a new swimlane inside the Backlog page.
6. Find the sprint you want to delete, and select ... > Delete sprint.

You can also use REST api to delete the Sprint:
https://developer.atlassian.com/cloud/jira/software/rest/api-group-sprint/#api-group-sprint