See the release notes: we now have experimental support for using any prefix shorter than the Linux default. We are aiming to eventually fully support (Tier 1) any prefix under 64 bytes long.

The dedicated user isn't really required; it just expects that /home/linuxbrew/.linuxbrew exists. That can just be a symlink to your home directory. But yes, creating that symlink requires root on most systems. Brew itself doesn't require root or that pathname; you can put packages anywhere, but then many will have to be built from scratch since the pre-built packages don't work. And you need bubblewrap installed (which requires root) to use the sandbox, but again that is optional.

All that said, when you are running without a sandbox installing to a nonstandard location, not everything works consistently. I run this mode all the time. But it has been getting better.

I always download the install script and change the directory to my user home directory, ~/.brew

The dedicated user account is optional.

It uses root once to chown its prefix directory /home/linuxbrew/.linuxbrew