I read that in Sitecore 9.1 the admin user is no longer 'b'. Is that accurate?
As per installation guide, in version 9.1 the Sitecore admin password needs now to be set when installing it.
"One of the settings that you must edit is SitecoreAdminPassword. If you do not specify the Sitecore administrator password in the script and leave the example value "SIF-Default" unchanged, a random password is generated for you. This password is written to the XP0-SingleDeveloper.log file"
you can find in the installation guide here
Hope this helps
That is true. Default password is set to "SIF-Default" which during installation is detected by SIF and SIF generates new / random password for you.
However, you can easily change that password back to "b" through User Manager if you are feeling nostalgic.
Mind that this could be a security risk! If you are planning to use this old password on dev machine, you are probably safe to change it to "b".