Sneezing - I see your point. Though still think it's easy to say that in isolation, that 'mum' and 'stepmum' are sufficiently different enough, but in the context of a long complicated post I think it can save a few seconds of thinking time for the reader to use the term bio/birth mum occasionally!
But, am trying to think about this in other ways too.
I think there's another reason I might use the word, but it's hard to explain. Something to do with responding to threads where it's the stepmum who is at the centre of it all, it's about her point of view and all the other bits of the family are auxilliary to her. She is the centre of her own world - I am NOT saying that means she is the centre of the family in real life, or she comes first or whatever, but from the point of view that she has a problem or something to discuss online, she is at the centre of it. So we're not talking about the mum of the stepchild as a mum in and of herself, but rather we're talking about her in relation to the step mother, who is at the centre of the post.
Or, we might be talking about stepmums and birth mums as general categories, and the differences between them. So again, for the sake of clarity in both those situations, I would use the term birth mum. Because I'm talking about what someone is in relation to a stepmother, not what they are in relation to their child. Because I want to be clear I'm not talking about mothers generally, but rather mothers who have separated from their partners and whose children now have stepparents - writing 'birth mum' for me does the job of saying all that, rather than typing out that whole sentence!
Again, I'm not sure I explained that very well, and doubtless some people will take offence, but am trying to think out loud and be honest. And will say again that I swear I have no axes to grind, don't hate the mother of my DSS, that is in no way any part of my motivation for using this term.