When created a new task in a Next Gen project, sprints from deleted projects appear as options in the sprint select picklist. There appears to be no way to delete those sprints.
There solutions suggested in the posts below do seem to work anymore:
Sprints are not specific to a Project. A sprint appears in any board — a single board or multiple boards — as long as the issues assigned to the sprint match the filter query of the board or boards.
If you want to delete the sprints which are no longer required, you may follow the below workaround.
You may repeat these steps until you delete all unwanted sprints.
Please note that delete is a destructive operation and there is no way you can get it back once deleted. So, make sure you are deleting only the ones are not being used in other boards.
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