Formula Fields and Triggered Flows
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.