What foods can replace salt (other than soy sauce)?

We must not fight with ordinary (sodium) salt, but supplement it with its antipode - potassium salt. In stores you can find salt with a reduced sodium content - there are up to 30% potassium with magnesium. As a last resort, in any chemical laboratory, you can ask for potassium chloride and add it yourself to ordinary salt. By the way, of the available and inexpensive potassium products, there is only a lot (!) In potatoes, which should not be neglected by poor people

When my father was losing weight (and he loves VERY salty food), he replaced salt with kelp powder. He said that it tasted rather strange, but also salty, and you get used to it quickly.

I have to disappoint you: soy sauce contains salt, and in rather large quantities. So if the goal is to eliminate salt from the diet (or minimize its use), accustoming yourself to soy sauce is hardly advisable.
And so, for example, in many East Asian cuisines fish sauce is used instead of table salt (nyok-mam, nam-pla). It is good for something - unlike soy sauce, it does not have such a pronounced taste, it is more difficult to overdo it with it. But again, salt is contained in it.