For the shorter reading hypothesis to be correct, someone would have had to destroy all of the manuscripts containing the “original” version of Matthew 28.19 and replace them with new ones with the longer reading.This is quite a conspiracy theory that requires a level of control that did not exist at that time.
However, if there was an early manuscript with even a slight variation, Bruce Metzger’s UBS 4th edition or his Commentary on the GNT would note it. We still have thousands of manuscripts, some of which date back to the fourth century (like Codex Sinaiticus and Codex Vaticanus).
Thus, a later, or even medieval manuscript, could preserve a very early reading.
Again, we have no evidence of an alternative version of Matthew 28.19 in any of these manuscripts.
For Christ also said, ‘Except you are born again, you will not enter into the Kingdom of Heaven.’…”“…And again, giving to the disciples the power of regeneration into God, he said to them, ‘Go and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Spirit.’…”“For the law of baptizing has been imposed, and the formula prescribed: ‘Go,’ He saith, ‘teach the nations, baptizing them into the name of the Father, and of the Son, and of the Holy Spirit.’ The comparison with this law of that definition, ‘Unless a man have been reborn of water and Spirit, he shall not enter into the kingdom of the heavens,’ has tied faith to the necessity of baptism.”“And when he who is baptized goes down into the water, he who baptizes him, putting his hand on him, shall say thus: Do you believe in God, the Father Almighty? Then holding his hand placed on his head, he shall baptize him once.
And then he shall say: Do you believe in Christ Jesus, the Son of God, who was born of the Holy Spirit of the Virgin Mary, and was crucified under Pontius Pilate, and was dead and buried, and rose again on the third day, alive from the dead, ascended into heaven, and sat at the right hand of the Father, and will come to judge the living and the dead? And again he shall say: Do you believe in holy spirit, and the holy church, and the resurrection of the flesh?