So if /etc/passwd is describing shells that are no longer installed what happens?
try and tell me :p
Should (or does?) pacman change passwd to the next shell in the /etc/shells list?
Pacman only control package installation, it doesn't control the configuration of the system., so not.
This kind of files cannot be managed (and should not be) by pacman, only some applications can deal with those files like passwd, passwd, chsh , useradd,...