This condition is not yet available in JWT Cloud, but you can implement it easily using the Jira expression condition.
As described in A general note about JWT for Jira Cloud, it is not possible to use the JWT field codes in conditions or validators. That is the reason why JWT Cloud cannot provide a one-to-one mapping of the fields which can be selected in this condition.
Create a Jira expression condition
Enter the expression
user != issue.assignee
