The main reason to “eject” flash memory is if it is in the middle of a write operation because they are rather long due to how entire pages must be erased and rewritten. Depending on how this write process is managed and how the hardware is constructed it can cause corruption from removing something the wrong way.
I’m not sure about how the larger distros work, but with something like OpenWRT, everything is running in RAM. I’m pretty sure that is how Live USB always works.
I’m pretty sure even the distros that can run from USB with persistence are running an immutable OS that runs from RAM and then using an extra partition on the USB drive for persistent storage. The immutable nature hints that it is likely running in RAM.