At least in PG, that is explicitly not the case, unless I'm misunderstanding:

Similarly, a PL/pgSQL loop of the form FOR x IN query LOOP .. END LOOP will never use a parallel plan, because the parallel query system is unable to verify that the code in the loop is safe to execute while parallel query is active.

https://www.postgresql.org/docs/current/when-can-parallel-query-be-used.html

At any rate, I feel like it's questionable design to have a user making row-by-row decisions on hard deletes.