The oldest surviving copies of the New Testament date to the 4th century, after Constantine declared Christianity the official religion of the Roman Empire. Of all the manuscripts made prior to that, only fragments survive.
For the Gospel of Matthew, the oldest surviving fragments are Papyrus 77, containing part of Matthew 23; Papyrus 103, parts of Matthew 13 and 14 (and possibly from the same maunscript as Papyrus 77); Papyrus 104, part of Matthew 21; and the Magdalen Papyrus (formerly identified as P64 and P67), a couple verses from Matthew 3.
The 4th century complete manuscripts (Codices Sinaiticus and Vaticanus) contain Matthew 28 but do not contain Mark 16:9-20.
So although it's true that we have no surviving evidence of Matthew 28 from prior to the 3rd century, we also have no evidence of it being omitted.