True today! In classical orchestra performances, that usually required the composer to insert a solo 'cadenza' section where the soloist was free to be expressive without a conductor 'click-track'.

Today, bands (esp. jam bands like the Dead) might agree to back up anyone who decides to go 'off-plan'. (These days, we can even take big samples of someone else's tracks (if they agree) and do 'covers' that are an entire re-working of their originals.)