The container was last updated Dec 20.
Plex is a rather unique situation in the unRaid universe. Limetech includes a default app for it, the majority of the users around here run the Plex app via either linuxserver.io or binhex's repositories, and now the guys over at Plex are now officially supporting unRaid with a template to their own docker app.
Because of this saturation for this particular app, if I had my way, then no more PlexMediaServers would be allowed into Community Applications as fundamentally they are all identical. Whether or not Limetech is going to continue to update their container now that Plex has an official docker for unRaid I do not know (EDIT: jonp just answered this above), but generally within the forum itself the advice has always been to install either the lsio or binhex flavours. With Plex officially supporting unRaid, I believe that gradually the advice from the forum will be to install the official version instead of any of the others.