Is there any historical reason for this? do any languages use "j" to indicate the plural?
In praesperanto-1878, the ending -s of the article and nouns was used to denote the plural. The form las nacjes was recorded; in modern Esperanto it is la nacioj; it can be assumed that a similar ending was used for adjectives as well. Obviously, this ending was borrowed from many languages: in English it is boys, in French it is garcons, in German it is Jung(en)s, in Spanish it is niños; in Latin it is pueros, in Greek it is παιδες. It should be noted that Zamenhof later criticised the overuse of this ending, as the sound [s] gave "too sharp a sound to the language". In the 1881-82 projects, -s was still retained as the plural for Feminine Nouns (who had the ending -aŭ: princaŭs "princess", amentaŭs "loving [women]"), but most nouns and adjectives were already used with the ending -j (maj revoj "my dreams", kalaj guroj "warm countries"). The final version of Esperanto included the singular ending of the plural -j.
According to the first version, abandoning the ending -s (see above; moreover, consonant 's' entered the verb endings -is, -as, -os and -us), Zamenhof chose another more or less common ending for the plural, namely -i (used in Italian, Latin and Slavic; cf. pueri, ragazzi, chłopcy). In order to avoid lengthening words into a whole syllable, Zamenhof chose to replace the extra -i with -j, getting the combination of -oj and -aj instead of *-oi and *-ai. Thus, according to Scott's wording, "vowel i was adopted in the semi-vowel form of -j".
Many researchers consider as the main "Greek version": in ancient Greek the ending -οι (similar to the Esperante diphthong -oj) is common for the plural of nouns of the second declension (the first declension uses the ending -αι): κοῦροι 'boys', ἄνθρωποι 'humans'; cf. κόραι 'girls'. This similarity is even more noticeable, for example, in circulation αἱ ἱεραί βίβλοι = la sanktaj libroj ("sacred books"). Another support for this version is the presence of combinations -οιν and -αιν (similar to Esperant -ojn and -ajn), which in Ancient Greek formed forms of genitive and dative cases of a rare double number: τοιν ὀφθαλμοῖν "[from] the eyes", οἴκίαιν "[from] two houses".
Source. Translated by robots, so feel free to correct the translation.
My non-scientific guess is that -j- is easier to combine with another consonant to mark the accusative, for example in “mi manĝas pomojn”. If s were chosen something like “mi manĝas pomosn” would be hard to pronounce and Zamenhof would have had to have thought of another way to mark the accusative on plurals. That might end up making the language less regular.