The Cambridge Bible notes for John 3:13 says:
which is in heaven] These words are omitted in the best MSS. If they are retained, the meaning is ‘Whose proper home is heaven.’ Or the Greek participle may be the imperfect tense (comp. Joh_6:62, Joh_9:25, Joh_17:5), which was in heaven before the Incarnation. It is doubtful whether in this verse we have any direct allusion to the Ascension, though this is sometimes assumed.
So if those words are not in the best manuscripts that we have, that is why more modern translations omit them, such as:
(TLV): No one has gone up into heaven except the One who came down from heaven—the Son of Man.
(ESV): No one has ascended into heaven except he who descended from heaven, the Son of Man.
(GNB): And no one has ever gone up to heaven except the Son of Man, who came down from heaven.
(NLT): No one has ever gone to heaven and returned. But the Son of Man has come down from heaven.
(NIV): No one has ever gone into heaven except the one who came from heaven—the Son of Man.
(CSB): “No one has ascended into heaven except the one who descended from heaven — the Son of Man
(NET): No one has ascended into heaven except the one who descended from heaven - the Son of Man.
(RSV): No one has ascended into heaven but he who descended from heaven, the Son of man.
etc.