There may be potential uses for the tag in the future, though I can't think of good examples. Should we have the tag?
Despite its existence on other sites, soft-question reeks of being a meta tag to me.
The physics site has this description for soft-question:
Questions that ask about some aspect of physics research or study which doesn't involve the actual physics. In general, soft questions can be answered without using physical reasoning.
What I see here is that they use this as a catch-all for subjects peripherally-related to physics, but the tag still doesn't give any information about the subject itself, except that it is not related to the actual physics theory.
As you noted, the question which you linked to was closed as too broad, and I'm guessing we'll see the same conclusion on similar questions. However, we have also discussed questions on not-specifically-engineering subjects like liability that have been decided to be in scope and acceptable if they are done properly. These would probably be good for a soft-question tag, but the same information could be conveyed by using a tag like liability. This implies that the question probably won't involve formulas and calculations, while also making a statement about the actual content of the question.
And as has been noted in the comments, I have another answer on meta tags on this question, which links to some good resources for the StackExchange site as a whole.