The illusory script spell allows one to hide a secret written message that either appears unintelligible or appears to be an unrelated message to anyone other than the intended recipient(s). The spell explains clearly what happens if the spell is dispelled:
Should the spell be dispelled, the original script and the illusion both disappear.
This is certainly a handy security feature: dispelling the illusion won't reveal your secret message to snooping spellcaster. However, the spell is less clear about what happens at the end of the 10-day duration. A spell that ends due to reaching its duration is not "dispelled", so the above clause would not apply directly. The only reference to the spell's duration is in the first sentence:
You write on parchment, paper, or some other suitable writing material and imbue it with a potent illusion that lasts for the duration.
So, what happens when the duration of illusory script elapses? Do the illusory script and the real message both disappear, as when the spell is dispelled, thus ensuring that no one other than the intended recipient(s) can ever read the message (without truesight)? Or does the illusion disappear, leaving the real message plainly readable by anyone?