As your last-cited Google page demonstrates, the iota subscript is usually ignored in modern spellings of the adjective formed from her name in Greek. However, when the name of Leto was made into an adjective (to mean, e.g., "of Leto" or "[shrine, temple] of Leto") in ancient Greek, the ending -ιος was appended, resulting in epic/Ionic Λητώιος, or, with the iota subscripted in Attic/koinē, Λητῷος. The grammarian Aelius Herodianus (2nd cent.) says it best (De prosodia catholica 3,1, page 122, line 23):
τὰ δὲ εἰς <ιος> τὸ <ω> κατὰ τὴν τρίτην συλλαβὴν ἔχοντα ἀπὸ τῶν διὰ τοῦ <ῳος> κατὰ διάλυσιν γίνονται, Κῷος καὶ <Κώϊος>, Μινῷος καὶ <Μινώϊος, λώϊος, πατρώϊος, ἡρώϊος, Τρώϊος, Λητώϊος>.
Any appearance of the adjective in an ancient text without the iota, either adscript or subscript, would be, at best, a typographical error.