From https://manpages.debian.org/wheezy/multistrap/multistrap.1:
--no-auth - allow the use of unauthenticated repositories. Same as noauth=true
What I understand from this definition is that multistrap won't try to authenticate, so unsafe software might be installed.
However it doesn't seem to be the case: Setting noauth=true throws the following error:
Get:1 http://ftp.uk.debian.org/debian buster InRelease [122 kB]
Err:1 http://ftp.uk.debian.org/debian buster InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
Reading package lists... Done
W: GPG error: http://ftp.uk.debian.org/debian buster InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 04EE7237B7D453EC NO_PUBKEY 648ACFD622F3D138 NO_PUBKEY DCC9EFBF77E11517
E: The repository 'http://ftp.uk.debian.org/debian buster InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
apt update failed. Exit value: 100
Interesting part is, setting noauth=false fixes the problem.
What does noauth cause exactly?