Starting with the release of Strategy ONE (March 2024), dossiers are also known as dashboards.
Option to use thresholds doesn’t appear for custom groups in Strategy 10.4 and above.
1.) Create a Dossier and add an existing dataset.
2.) Drag an attribute into the visualization and right-click on the attribute. Notice that applying thresholds is an option. Right-click one of the attribute elements and choose the “Group” option. Name the group.
3.) Right-click on the attribute and notice that the option to apply thresholds is removed.

The functionality is working as designed. The ability to use thresholds for attributes was included in Strategy 10.4 as an enhancement. This enhancement doesn’t include the same functionality with custom groups.
A separate Enhancement Request has been logged for this functionality.