Pluggable Authentication Modules (PAM) is a (not really) newer generic API for authentication originally proposed by Sun Microsystems in 1995 and adapted a year later on the Linux ecosystem. With an API change, don't expect things to keep the same name (nor to always map exactly the same).
from man pam_unix:
OPTIONS
[...]
minlen=n
Set a minimum password length of n characters. The default
value is 6. The maximum for DES crypt-based passwords is 8 characters.
Implementation can vary from distribution to distribution. On Ubuntu (like on Debian), /etc/pam.d/passwd will source /etc/pam.d/common-password where the first password occurence includes the useful options. It can be altered and have an minlen=X parameter added. This later file is autoregenerated by pam-auth-update typically on upgrades, so check it works after an upgrade of PAM-related packages. Normally it should:
The script makes every effort to respect local changes to
/etc/pam.d/common-*. Local modifications to the list of module
options will be preserved, and additions of modules within the managed
portion of the stack will cause pam-auth-update to treat the
config files as locally modified and not make further changes to
the config files unless given the --force option.
Other options might still affect the result. Eg, if you set minlen=1, the obscure option will still reject a one-letter password as a palindrome, or a 2-letters password as too simple etc.
Be careful when altering PAM: you might lock yourself out of your system in case of wrong settings.
