That’s more of a licensing question. Even if you’re personal use of watching videos on YouTube is covered, it’s a difference if Plex as a company starts re-distributing them (even re-packaging them in their platform).
Also… Plex have started to reject requests to integrate certain providers.
From Please Read: Before submitting a feature Suggestion
Requesting integration of content/service providers, and other potential partner systems
We do not have any control if a content provider wants to, has a compatible API or if they are allowed based on their contracts with content creators to resell the streaming rights to said content.
While in the past we left these content provider feature suggestion topics open as a reminder if one did get back to us, it is clear that they generally just become toxic with false beliefs and accusations about what we can or cannot do.
Any suggestions for integration of a content provider will be closed. If you wish for a content provider to be be a partner and integrate with us you should contact that provider and suggest they contact us . If they are able to both legally and technically have their service integrate into our product we would happy to explore how to potentially integrate their service.
I’ll therefore close this suggestion.