Of ἄρρητα, arreta, Strong 731 Thayer states :
a. unsaid, unspoken: Homer, Odyssey 14, 466, and often in Attic.
b. unspeakable (on account of its sacredness) (Herodotus 5, 83, and often in other writings): 2 Corinthians 12:4, explained by what follows: ἅ οὐκ ἐξόν ἀνθρώπῳ λαλῆσαι.
'Unsaid' or 'unspoken' does not convey that something cannot be uttered because it is impossible to do so. Rather, something remains unsaid because people do not like to say it.
'Unspeakable' is stronger and the reason given is because the subject is sacrosanct. The reluctance to speak about it is unanimous, because people fear to broach the subject.
As for ἐξὸν, exon, Strong 1832, it is the same word that Jesus uses in Matthew 12:4 regarding the shew bread which it was 'not lawful' to eat (according to the law).
Thayer's note states that the reason for the matter being 'unspeakable' is because of what follows, that is to say, because of the subject being unlawful to be spoken of.
Paul's statement indicates that it is just not right to talk about such things. They are to be kept to oneself, if one has had such things revealed to oneself.