Does freezing non-freezable liquids affect life span
Most sauces do freeze solid, but it is the temperature, not the solidity, that matters for food safety.

The hardiest bacteria, temperature-wise, are psychrophiles and can grow in temperatures as low as -15° C. Since they can't grow in temperatures above 10° C, they are very rarely found in food, but if your freezer is actually at -18° C (and you've verified this with a thermometer) then the food is quite safe.

Practically speaking, most of the common foodborne bacteria are mesophiles (Salmonella, E.coli, etc.) and don't grow well below 20° C. It's impossible for any of these to grow at even a "warm" freezer temperature.

Bottom line: Check your freezer temperature (and, if you're really paranoid, the food) with a thermometer to make sure it's working properly, but otherwise, don't worry about it.

P.S. Vinegar lowers the freezing point slightly, but more importantly, sugar causes the other liquids to freeze into much smaller crystals, which may give the appearance of not really being frozen (think of "soft" ice cream). Plum sauce also tends to contain various gums such as xanthan gum as thickeners, which are also used to keep commercial ice cream soft.