As VikingOSX states it, the default PATH has been modified. This is unfair, as it wasn't mentioned anywhere.
Moreover, the ftp itself, which was in /usr/bin, has been removed: I installed High Sierra this morning, so my locatedb is as yesterday, and shows it was still there before upgrade:
$> locate ftp | grep ftp$
/Applications/ Développement/Xcode.app/Contents/Developer/usr/libexec/git-core/git-remote-ftp
/Applications/GitHub.app/Contents/Resources/git/libexec/git-core/git-remote-ftp
/System/Library/Filesystems/ftp.fs/Contents/Resources/mount_ftp
/System/Library/Tcl/tcllib1.12/ftp
/sbin/mount_ftp
/usr/bin/ftp
/usr/bin/sftp
/usr/bin/tftp
/usr/share/zsh/5.2/functions/_ncftp
/usr/share/zsh/5.2/functions/_zftp
/usr/share/zsh/5.2/help/zftp
but it you try to make an ls (LS) on this list, then /usr/bin/ftp is missing, as some zsh parts:
$> ls -ld $(locate ftp | grep ftp$)
ls: /usr/bin/ftp: No such file or directory
ls: /usr/share/zsh/5.2/functions/_ncftp: No such file or directory
ls: /usr/share/zsh/5.2/functions/_zftp: No such file or directory
ls: /usr/share/zsh/5.2/help/zftp: No such file or directory
lrwxr-xr-x 1 root wheel 15 23 sep 19:56 /Applications/ Développement/Xcode.app/Contents/Developer/usr/libexec/git-core/git-remote-ftp -> git-remote-http
-rwxr-xr-x 1 myusername staff 850620 30 jui 2015 /Applications/GitHub.app/Contents/Resources/git/libexec/git-core/git-remote-ftp
-rwxr-xr-x 1 root wheel 1905072 14 nov 2012 /Applications/SourceTree.app/Contents/Resources/git_local/libexec/git-core/git- remote-ftp
-rwxr-xr-x 1 root wheel 31616 21 sep 06:37 /System/Library/Filesystems/NetFSPlugins/ftp.bundle/Contents/MacOS/ftp
-rwxr-xr-x 1 root wheel 18224 21 sep 06:34 /System/Library/Filesystems/ftp.fs/Contents/Resources/mount_ftp
drwxr-xr-x 5 root wheel 170 16 jul 02:02 /System/Library/Tcl/tcllib1.12/ftp
drwxr-xr-x 82 myusername wheel 2788 30 jui 20:09 /Volumes/TMBKIMAC/BkAppsIMac/
lrwxr-xr-x 1 root wheel 63 30 sep 02:08 /sbin/mount_ftp -> /System/Library/Filesystems/ftp.fs/Contents/Resources/mount_ftp
-rwxr-xr-x 1 root wheel 110720 21 sep 06:35 /usr/bin/sftp
-r-xr-xr-x 1 root wheel 39008 21 sep 06:35 /usr/bin/tftp
I bet that I'm going to get the /usr/bin/ftp in some backup, and check my paths to fix them in POSIX way.
p.s. Not related to High Sierra but seen while writing this: if one clicks here on "Use advanced editor" while replying, then there is an error since we have no permission to do so. Please tell the webmaster to have this fixed asap. Found 'cause I wanted to set another style for terminal excerpts.
HTH.