There really is no need for an AuthShadowFile directive. The purpose of a shadow file is separate sensitive information (e.g. passwords) from other account information (username/UID/GID, etc). Programs like /bin/ls often reference the passwd file in order to display user/group names rather than numbers; these programs do not really need that sensitive information. Rather than relying on programs like /bin/ls to ignore the sensitive information, libraries were developed to split the information into /etc/passwd, /etc/shadow (and similarly for /etc/group, but very few administrators use group passwords anymore). Some operating systems, most notably FreeBSD, though, chose a different form of information separation. Since FreeBSD maintains account information in binary database files, the shadow libraries mentioned above are not used. Instead, FreeBSD returns the sensitive information to the calling program only if it has sufficient (i.e. superuser) privileges.
When proftpd uses an AuthUserFile, it is looking for all of the account information, including the password. And since AuthUserFiles are specific to proftpd, there is no need to split any passwords out of an AuthUserFile into an AuthShadowFile. As the documentation states, an AuthUserFile need not reside inside a chroot() filesystem, which means that users can be effectively isolated from having access to that AuthUserFile. At that point, the only consideration is making sure that the permissions on the AuthUserFile are sufficient for the server to have access, but no other users.