The special stock indicator is now available since CE2108 to be used as a selection criteria for IM output determination. You are unable to find the new field to use it.
SAP S/4HANA Cloud 2108
Reproducing the Issue
1. On Output Parameter Determination App, check rules for Goods Movement, Output type.
2. Click on edit mode > choose table settings > insert from context data object.
3. Field SPECIALSTOCKTYPE is not available for selection.
The reason why your field is not available while adding a column at the decision table is because you need to refresh condition parameters of the application.
In order to Refresh Condition parameters please follow Output Management Setup Instructions, in chapter 188.8.131.52.2 Refresh Condition Parameters.
Field, Special, Stock, Type, Output, Parameter, Determination,Table, Settings. , KBA , CA-GTF-OC , SAP S/4HANA output management , How To