I think it's about rewiring very very strong belief mechanisms by having small impact events again and again ( having small conversations with people here and there in public) or having a very strong event where your belief system is completely altered ( Like the one where someone does large scale public speaking or just dives into the deep end by taking a job where u speak all the time). Also exhausting the fear system, works too. Like try sitting with one person at work on a project and just work all day with the person. By the end of the day, I bet your nervous system is so tired of being scared of fucking up and stuttering, that it just says " fuck it, I don't care anymore". Trauma plays and has played a very heavy role in my stuttering. Being beaten ruthlessly by my parents and growing up with a lot of abuse sure impacted my stuttering even if it wasn't the main cause.