Pretty sure the sugar is necessary for preserving. You could make it without sugar, but it would just be pulpy juice-water (the sugar is also a thickening agent), and you would have to eat/drink it pretty fast because it will go bad quickly (I would guess a couple days)
In a sterilized and sealed jar, jam made without sugar can last for years. Once you actually open the jar, you have about 7 days to eat it, and you better keep it refrigerated. You don't need the sugar for thickening - the pectin in the fruit thickens jam just fine.
However, if you don't add any sweetener, the result will be very sour.
Source: been making my own jam for years, had plenty of time to experiment.
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