Unfortunately, there is no native function available in JIRA Cloud to do this. I haven't been able to find a Cloud add-on that can help. Maybe someone else in the community will be able to suggest something, better but...
..a possible workaround is JQL search ordered by sub-tasks, so that issues with no sub-tasks will show first in the results:
issuetype = Task ORDER BY subtasks DESC
Add the "Sub-tasks" column to your filter results so you can see which have none:
I found a couple of suggestions in Atlassian's public issue tracker that could be worth watching/voting on:
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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