I didnt have selective mutism, but sign language was my first language so often as a child, and to a degree still now, when extremely stressed or anxious or shy I couldnt speak verbally at all, but I would communicate still through sign language.
Throughout middle and high school a had a handful of ears I would talk off, and everyone else I would not say a single word to. I'm still not comfortable talking with most people, but back then I said zero words to most people. I would actually say I still don't say a lot, but the difference is back then I was actively trying not to say words, whereas now I may be in a conversation and just have no words to say. If anything conversation came more naturally when I was younger and was trying to avoid it.