Does Sirach 12 ('do not give to sinners') contradict Jesus in Matthew 5, or is it supersessionism?

There is a totally different context. Matthew 5 is about the law and how the hypocritical Jews of that day had confined it, keeping the letter, thou shalt not kill, while insulting and belittling their neighbor, and Jesus says even calling a man a fool makes you liable to judgment, so the law is far more comprehensive than the letter. Sirach 12, by contrast, is talking about being careful to whom you do good, which is exactly what Jesus says in Matthew 7 just after the Sermon on the Mount: do not cast your pearls before swine. Be careful whom you give your alms to, because they may spit in your face. The Christian is not to be a doormat; we should be open and merciful to everyone, but if we get a sign, and it is often easy to pick up, that the person will not appreciate the kindness or will try to hurt us, we are not obligated. Jesus himself was very careful whom he dealt with, wise as a serpent and gentle as a dove. So it is not supersessionism; Sirach and Matthew are making complementary points.