What authentication? If you just use it in your local subnet you don't need to log in. I've been using the free version for years and I do not have a Plex account.
They've "streamlined" it to the point of unusability.
That version is open sourced (https://github.com/plexinc/roku-client-public). The Plex developers have abandoned it, for updating the newer client only; but it's still perfectly usable as the old-version.
source: I wrote skip-sign in, manual server entry in the core platform.
It also works with a Raspberry Pi client without authentication.