This would be a subset of a total population that is affected by a change in policy or law. I tried 'party' but that really didn't work and sounded too legalese. Phrases like 'impacted persons' or 'affected persons' were cumbersome.
Impact is a synonym of the word affect. In some dictionaries, it's called "affectee." It's arguable, however that it is a valid word.
Technically, somebody who performs an action can be called an agent and somebody who is the object of that action can be called a patient:
2 : one that is acted upon
In linguistics, when a sentence is analyzed, the words agent and patient are used rather than subject and object.
However, even if correct, I doubt it would sound natural enough for your purpose. (If you object to other connotations of subject you'll have similar objections to patient.)
Other variations that imply a "role" could be the changed or the affected, but those are not typical uses of the words and adding the definite article in front of them could be considered a kind of cheat.
Target need not refer to somebody who is negatively affected:
b : something or someone to be affected by an action or development
However, it commonly is associated with something negative.
You may be forced to have to use a different word in different specific situations. For instance, somebody who receives something is a recipient or a beneficiary, and somebody who is hurt is a victim.