You can find it in the "Mail for domain" section, at the bottom.
Whenever an option “disappears”, it’s usually been moved into Server Templates (which allows the option to be different for different users or resellers).
The sub topic like
"Shell for mailbox users"
"Shell for FTP users"
"Shell for jailed FTP users"
"Shell for domain users"
"Format for usernames that include domain"
May move to other session as "server templates" side. Seems this document written with v2.60 interfaces. So after update to v3.X. Style change. we will not find the above setting area in Modules-> Defaults for new domains