Guys the app hasn't been updated is over a year. It's not going to get one to fix this. Let it go.
If the team are not going to support the app then they should pull the app from the store and stop advertising their paid product as supported by Xbox360: https://www.plex.tv/apps/tvs-consoles/xbox-360/
There's literally over 20 clients. Priorities change. Are you really surprised? Maybe you should buy an Xbox One?
Plex chooses to advertise support that is not really there.
The fact that plex is also supported on other clients is not all that relevant.
If I buy RAM from qualified vendor list it should work, right? And if it didn't, I would have grounds for a complaint and refund.
The plex app on Xbox 360 does not work as advertised. As a consumer, I have no obligation to understand the technical/manpower challenges involved in maintaining the product, I can simple expect an adherence to truth in advertising.
I won't bother petitioning the Dev. team, I'll just make a complaint to the FTC.