This last volume focuses on other's life after the incident and things left by Korosensei to the students. It begins with the graduation day and what happens to the school after the graduation.
The story even goes further about their life after seven years. Everyone will never forget Korosensei and promise to live their life well.
More reviews can be found on Goodreads: Assassination Classroom, Vol. 21: Time to Say Thank You (Assassination Classroom #21).