[slededit]

Calculators automate at least some "mental effort" and have existed since 2000BC. Perhaps you mean creativity? But even in that case automating composition of music was experimented with as early as the 1960s.

Certainly our ability to automate is much more refined than in the past, but I fail to see the fundamental shift you describe.

[detaro]

If you count variants of https://en.wikipedia.org/wiki/Musikalisches_W%C3%BCrfelspiel, "composition" with less mental effort is even older.