While being able to physically reproduce the exact sound is reasonably new, there are large numbers of examples from classical music that are explicitly and on purpose borrowing themes from other composers and altering them.
E.g. Bach's Canonic variations on "Vom Himmel hoch da komm'ich hier" from 1747 (based on a hymn written and composed by Martin Luther).
"Theme and variation" is a formal technique in classical form, that while it does not require borrowing (the theme could very well be your own) has also been extensively used by classical composers to borrow other composers themes for at least 250 or so years and weave it into their own music.
While being able to physically reproduce the exact sound is reasonably new, there are large numbers of examples from classical music that are explicitly and on purpose borrowing themes from other composers and altering them.
E.g. Bach's Canonic variations on "Vom Himmel hoch da komm'ich hier" from 1747 (based on a hymn written and composed by Martin Luther).
"Theme and variation" is a formal technique in classical form, that while it does not require borrowing (the theme could very well be your own) has also been extensively used by classical composers to borrow other composers themes for at least 250 or so years and weave it into their own music.