No, Matthew 18:8 is still discussing stumbling blocks (see Matthew 18:7)—moving from causing others to stumble to receiving or experiencing stumbling. Jesus had already said these same words to his disciples (Matthew 5:29–30), not literally meaning the cutting off of a hand or foot, or the plucking out of an eye, but rather that even the most precious possession must be given up if it leads one into sin. Matthew 18:8–9 is not about church discipline, except insofar as this passage speaks of the seriousness of causing others to stumble, so that the church must remove from its midst whoever causes offense. The fact that this follows only by inference from this passage, however, is seen in the fact that, when speaking of discipline, the subject has changed—namely, not the church, for this passage concerns individual believers.
No, Matthew 18:8 continues the theme of stumbling blocks (see Matthew 18:7). However, whereas the preceding passage had dealt with causing others to stumble, this one concerns receiving or experiencing stumbling. There is, in fact, a double metaphor here: Jesus had already spoken these same words earlier to his disciples (Matthew 5:29–30). At that time, it became clear that the cutting off of a hand or foot, or the plucking out of an eye, could not be taken literally. What Jesus meant was this: that even the most precious possession must be given up if it leads one into sin.1
Matthew 18:8–9 therefore does not speak about church discipline, which can only be inferred indirectly—that is, in connection with what had been said elsewhere, and insofar as this passage speaks of the dreadful seriousness of causing others to stumble. From this, one may deduce that the church must now remove from its midst anyone who causes offense. The fact that this follows only by inference from this passage, however, is evident from the fact that, when speaking of discipline, the subject has changed—namely, it is not the church, for this passage addresses the individual believer.2
8 And if your hand or your foot causes you to sin, cut it off and throw it away. It is better for you to enter life crippled or lame than with two hands or two feet to be thrown into the eternal fire.