Inactive User

@gurdeep did you try using getdeactivatedusersby(......) function, as per the documentation this "Retrieves all the deactivated users matching the given criteria"

Hi, you can use IsUsernameTaken(). If it helps, I have created an expression rule that checks whether a user exists or not and correspondingly returns the username or null based on the result. Feel free to refer this.
if(
rule!APN_isBlank(
ri!user
),
null,
if(
IsUsernameTaken(
ri!user
),
user(
ri!user,
"firstName"
) & " " & user(
ri!user,
"lastName"
),
ri!user
)
)

Thanks guys, my issue is resolved.

The 'status' property in the OOTB function fn!user() should be enough to check the active/inactive status of the User object.