It’s eth, actually, not thorn.
I had thought that eth was used in Old English for the voiced “th” and thorn for the unvoiced “th”, but Wikipedia says they were used interchangeably for both sounds.
You’re right otherwise. Thorn was not available on printing presses because they were being made in countries that didn’t use the letter, which is why the letter Y was used instead until “th” became more common.
As a kid, I had a small dictionary, so I checked whether “gullible” was in it in order to mess with my little sister. It wasn’t there, but I still got in trouble when my mom overheard me telling my sister, “Did you know gullible is not in this dictionary?”
I was miffed because I was telling the truth.