Linear Elamite — also called “Old Elamite” — was the outgrowth of proto-Elamite in modern-day Iran.
We tend to think of languages as either “deciphered” or “undeciphered”, but Linear Elamite is currently partially deciphered and likely to stay that way until more artifacts are uncovered.
It has been difficult to decipher because there are only twenty-two texts as of this writing. In addition, Elamite appears to be a language isolate. However, some of the artifacts have long sets of text in both Linear Elamite and Akkadian cuneiform, which helps.
There are about 80 glyphs in the twenty-two existing texts, which is more than most syllabaries but way less than most logographic languages. It is likely that it was mostly syllabic, but with some logograms here and there.