Nowadays, if you wanted something you could boot off a USB flash drive, you'd use MX Linux. It even supports loading the entire OS into System RAM so you can eject the USB flash drive after it has booted.
Persistence is optional here, you can either have it or not have it.
It also has a built-in tool to remaster the OS image, so you can update all the packages, install a few more, then run a Remaster and then you have a brand new USB bootable OS image with updated packages.
MX Linux also has the "Frugal Install" feature that lets you install the USB version of the operating system to your hard drive, but it will still act just like you booted from USB, with the system being rolled back if you don't manually persist the system.
Persistence is optional here, you can either have it or not have it.
It also has a built-in tool to remaster the OS image, so you can update all the packages, install a few more, then run a Remaster and then you have a brand new USB bootable OS image with updated packages.
MX Linux also has the "Frugal Install" feature that lets you install the USB version of the operating system to your hard drive, but it will still act just like you booted from USB, with the system being rolled back if you don't manually persist the system.