The phrase ”throughout your generations” in Gen 17:9-14, means not just this generation, but generations to come. The generations referred to in Gen 17:9, are the Jewish people. In the context of the covenant of circumcision “throughout their generations” is ended by the coming of the promised Messiah, ie Jesus. The circumcision that both Jewish and gentile believers now require, is to put off the flesh. In other words, we must die to serving sin and live to God.
Rom 6:11-13 So you also must consider yourselves dead to sin and alive to God in Christ Jesus. (12) Let not sin therefore reign in your mortal body, to make you obey its passions. (13) Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness.
So circumcision is now of the heart, or symbolic, not literal. Paul explains this in Colossians 2:
Col 2:11-14 In him also you were circumcised with a circumcision made without hands, by putting off the body of the flesh, by the circumcision of Christ, (12) having been buried with him in baptism, in which you were also raised with him through faith in the powerful working of God, who raised him from the dead. (13) And you, who were dead in your trespasses and the uncircumcision of your flesh, God made alive together with him, having forgiven us all our trespasses, (14) by canceling the record of debt that stood against us with its legal demands. This he set aside, nailing it to the cross.
This is a much more meaningful thing than physical circumcision. In Galatians 5, Paul emphasises the fact that physical circumcision is no longer required:
Gal 5:2-6 Look: I, Paul, say to you that if you accept circumcision, Christ will be of no advantage to you. (3) I testify again to every man who accepts circumcision that he is obligated to keep the whole law. (4) You are severed from Christ, you who would be justified by the law; you have fallen away from grace. (5) For through the Spirit, by faith, we ourselves eagerly wait for the hope of righteousness. (6) For in Christ Jesus neither circumcision nor uncircumcision counts for anything, but only faith working through love.
So Jewish people today who are not literally circumcised will not be “cut off from his people”. However, if they do not accept salvation on God’s terms, through Jesus Christ, they will eventually be cut off.