Earlier quoted context omitted.
> every program you install gets full access to the system It gets your user's privileges when you run it. It's up to you how you run your programs. Also there's no "default Linux configuration". There are many distributions, and they have different defaults and approaches.
Many distros with many approaches, yet not a single one with a convenient security feature that comment is mentioning So it's not "up to me" if the good choice is not practical
Yeah, it's too hard to just `useradd -m name` a new user, maybe set default acl once via setfacl -m d:u:main-user:rwx /home/name for easier file sharing with the main desktop user account, and `sudo su - name` to it, and run whatever less trusted apps need to run under that user account from then on, mostly isolated from the rest of the [file]system.
Distros clearly don't allow this and none has this feature or these commands preinstalled by default, nor they are built to be multi-user OSes. :D