Perhaps here frappe/toolbar.js at develop · frappe/frappe · GitHub
That file turns up from this search:
(env) frappe@ubuntu1804lts:~/frappe-bench$ find . -name '*.js' | xargs grep 'Request Feedback'
./sites/assets/js/form.min.js: this.page.add_menu_item(__("Request Feedback"), function() {
grep: ./apps/frappe/node_modules/big.js: Is a directory
grep: ./apps/frappe/node_modules/highlight.js: Is a directory
grep: ./apps/frappe/node_modules/ipaddr.js: Is a directory
grep: ./apps/frappe/node_modules/fuse.js: Is a directory
./apps/frappe/frappe/public/js/frappe/form/toolbar.js: this.page.add_menu_item(__("Request Feedback"), function() {