If you ask me, it's always been a bad idea. I'm pretty sure the only reason we have it is historical; it's one of those tags that gets created on every new site right in the beginning, before we figure out what kinds of questions we want. But these days, as you've noticed, pretty much anything that deserves the tag soft-question turns out to be off topic.
I'd be in favor of blacklisting or burninating (technical term) the tag so that new posters can't use it anymore. Of course, if we do that people will probably continue to post those questions and they won't be so easy to find, but for organizational consistency, we probably shouldn't have a tag whose questions are so often off topic.
If we don't want to do that, SE is starting to experiment with tag warnings on other sites: little popup boxes that display some warning text whenever someone tries to use a particular tag. Perhaps we could request a tag warning for soft-question which points out that questions with the tag are likely to be off topic, and linking to a meta post (to be written) giving guidance on what sorts of soft questions are likely or unlikely to be okay.