This appears to be a Core Board on Jira Cloud. These specific types of boards do not have a means to configure the issues that appear here like you can do with a Jira Software Board. On a software board you can change the JQL filter that determines what issues appear there, but there is no such mechanism available to configure with a Core Board. More information on the different types of boards in Jira can be found in Configuring Boards.
It appears that Core Boards have never had this particular feature of showing subtasks, but I can see why this would be useful to have. As such I created a feature request for this over on:
I would recommend watching, voting, and if you like leave a comment on that page to help explain why this feature would be useful to have in your own words.
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
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