Jodie Miners has written about all the ways Formula fields can be referenced in Triggered Flows in the Spring ’21 release.
As of Spring ’21 sandbox testing:
- Formulas CAN be used in trigger conditions on before save flow triggers
- Formulas CAN be used in trigger conditions in after save flow triggers
- Formulas CAN be used in decisions in before save flow triggers IF the Resource uses the Prior Value (eg {!$Record__Prior.IsActive__c}). But you can NOT use formula values from the newly changed record.
- Formulas CAN be used in decisions in after save flow triggers
- Formulas CAN be used as the source value in Assignment Updates in before save flow triggers IF the Value uses the Prior Value (eg {!$Record__Prior.IsActive__c}). But you can NOT use formula values from the newly changed record.
- Formulas CAN be used as the source value Update records elements in after save flow triggers
Read the full post with examples here.