The Pharisees had a debate concerning what would happen to the most wicked of people, annihilation or indefinitely long suffering in Gehenna. The Pharisees believed that when a person died, his soul went to either Ga Eden/Paradise/Abraham’s bosom or he went to Gehenna which was primarily a place of purification, though for the completely wicked they debated whether or not the person would continue to suffer their indefinitely long or be annihilated after 12 months of suffering. Mt.10.28 could allude to this, and Matthew does focus much of his Gospel on Jesus countering the teachings, attitudes, and practices of the Pharisees. So Jesus “could” be alluding to this debate and affirming that the most serious thing that God would consider doing as punishment for evil would be annihilation. But there is nothing in the literary context that indicates that Jesus is addressing this issue.
Of course, the Pharisees and Sadducees and the common people would have known that Gehenna was a place of judgment of Israel, where dead bodies were cast when Jerusalem was destroyed. So this could be an allusion to that also, but again, the literary context is not about that.
In its literary context, Jesus is warning the apostles that they will likely suffer terrible persecution, and encouraging them that they should not fear what man can do to them. Rather they should keep in mind two things. 1) God is ultimately the one that is in control of who lives and who dies. If they are going to “fear” anything, it is God that they should fear, not man. And 2) they should be filled with faith knowing that God loves and cares for them! To extrapolate this passage and make it say more than this is not warranted, imo. And to then take this passage and use it to “prove” or affirm “Hell” is misusing this scripture.