What I suspect is the case with supermarket pickled onions is that the onions are immersed in a salted vinegar solution, hence a salt content. That might be the commercial method of production and it would also explain why they don't taste as good as home-made pickles. However, I've never looked at the contents of supermarket pickled onions, so I'm just speculating.
Home-made pickled onions don't have any salt, unless you fail to rinse them properly after brining, in which case there would be a trace. Let's face it, anyone who can't rinse an onion properly shouldn't be left alone in such a dangerous place as a kitchen anyway.
Here's one of my favourite chutney recipes. No salt. Lots of flavour. I once 'lost' a jar and found it five years later; it was the tastiest chutney I had ever eaten!
Apple And Date Chutney
Makes 4lb
3lb cooking apples, peeled, cored & diced
2lb onions, chopped
12oz stoned dates, chopped
4oz raisins
2 lemons, juice & rind
1 1/2lb light soft brown sugar
1/2 pint malt vinegar
Heat all the ingredients in a saucepan until the sugar dissolves. Bring the mixture to the boil, reduce the heat and simmer uncovered for 1 - 1 1/2 hours, until the mixture has thickened. Spoon into sterilised jars and store in a cool, dark place for 3 months.