Yes. It is from a plex.tv address.
The support article: https://support.plex.tv/articles/235974187-enable-repository-updating-for-supported-linux-server-distributions/
A thread from January on the topic: https://forums.plex.tv/t/issue-upgrading-to-pms-1-43-0-10467-on-debian-and-rhel-based-distributions/935847