The headword in this precise form - the perfect passive participle in the feminine nominative plural - is attested only in lexicography; see esp. ps-Zonaras s.v., where the determining noun is revealed as ἡμέραι (days). The headword verb δημεύω more usually means to declare (something) public property, to confiscate (cf. delta 424), and the glossing verb ἀπαγορεύω more usually means to forbid, to renounce (cf. alpha 2866, alpha 2867); both verbs can, nevertheless, mean to announce publicly (see LSJ ss.vv.).
Andrés Rodríguez Cumplido points out that the neuter plural δεδημοσιευμένα occurs in Aristotle, Rhetoric 1395a20 (see web address 1), referring to "popular sayings." See also e.g. Plato, Philebus 14D-E.