To search the code try something like this
frappe@ubuntu:~/frappe-bench$ find . -name ‘*.js’ | xargs grep ‘changed value of’
That leads to this code here
https://github.com/frappe/frappe/blob/develop/frappe/public/js/frappe/form/footer/timeline.js#L560
https://github.com/frappe/frappe/blob/develop/frappe/public/js/frappe/form/footer/timeline.js#L663
To help with your learning best to check out the Version doctype.