Could you provide an example of a musical piece where the performer is allowed to improvise a cadenza ad libitum?

One example of a musical piece where the performer is allowed to improvise a cadenza ad libitum is Mozart's Piano Concerto No. 20 in D minor, K. 466. In this concerto, the cadenza towards the end of the first movement traditionally allows the pianist to showcase their improvisational skills within the structure of the piece.