Be good to hear others opinions on this. I think it depends on the person. Maybe if it was very obvious and they worked hard at it then it might be appropriate. My stutter got a lot better when I was a teenager. Now all my friends from college etc literally never hear me stutter. They didn't even realise I had one for a while and to be honest I preferred that. So if I was your colleague I'd probably prefer you to not mention it at all. Because then I'd feel like "the stutter guy" again, even though I've made so much progress. But in your colleagues case, if they bring it up naturally in conversation, then I'd say it's most likely appropriate.