As of Spring '21 sandbox testing:

Example Formulas

My example object is a Contract object (but a Custom object)

IsActive__c

OR( ISPICKVAL(Status__c ,"Active"),
ISPICKVAL(Status__c,"Re-Contracting"))

IsClient__c

OR(ISPICKVAL(Account__r.Type,"Client"),
Account__r.NumActiveContracts__c > 0)

(NumActiveContracts__c is a DLRS that counts the No. of Active Contracts on the Account).

Flow Examples

Formulas in Trigger Conditions

Formulas in Decisions

In Before-Save Flows use $Record__Prior

In After-Save Flows use $Record

Using $Record in Before-Save Flows gives this error

Gotchas:

Assignments

In Before-Save Flows use $Record__Prior

If IsClient__c = true then add the special condition, otherwise blank.

In After-Save Flows use $Record

Using $Record in Before-Save Flows gives this error

Gotchas:

So, use formulas in Flow Triggers IF you can be sure they are stable formulas and be very careful to test what happens when the formula value changes!