Only languages in which (a) the accusative is unmarked, (b) in addition when a non-accusative is dative-shifted it loses its nominal case marking, and (c) in addition the dative-shifted non-accusative gains verb-coding of its semantic case via dative-shift, will exhibit the direct object-only constraint on object relativization.
Standardized
IF relativization of objects is restricted to direct objects, THEN the direct object is unmarked and indirect objects lose their own marking when becoming direct objects but are coded on the verb for their semantic case.
Keywords
dative-shift, direct object, indirect object, relativization, verb-marking
Accusative object in the terminology of Givón means “direct object”.