I'm a submissions reader elsewhere, there's no 'slush'. I think if you're not concerned about the term, then it isn't a problem for you, but language always carries messages - who knows that better than writers? If anyone feels as though their work is seen as something not to be respected (and I mean, as work per se, not in terms of quality, which is a relative thing), then surely we can only encourage better standards if we use a term that implies a standard, rather than one that can be seen as dismissive? As to 'commissioned' - when work is commissioned in any line of business, it's always within certain parameters, it doesn't mean 'do this thing like this'. If I'm asked to write a story for a given outlet, I do it according to their framework, but it's my product. And yes, it's solicited.
PS Apologies for my earlier, rather peremptory comment - sudden kitchen priority meant I had to hit 'post'!