pambase installs /etc/pam.d/system-login containing the following line:

session required pam_env.so user_readenv=1

According to pam_env(8), user_readenv is deprecated for security reasons, so since upgrading to pam 1.5.0, my systemd journal is full of:

systemd[22414]: pam_env(systemd-user:session): deprecated reading of user environment enabled
lightdm[22409]: pam_env(lightdm:session): deprecated reading of user environment enabled
sshd[23040]: pam_env(sshd:session): deprecated reading of user environment enabled

I'm no expert on the matter and I don't know why this was enabled in Arch, but it's probably best to reconsider this.