Archived YouTube pages claim to be ending browser support
Those who use Scrivener's webpage archive feature to store YouTube videos will note that the content of the page has changed to state that Google will be ending support for "this browser".
Scrivener makes use of the operating system's WebArchive format for storing and displaying archived pages. This is the same type of file you will get when saving a page from Safari, using default settings.
The WebArchive format has always been somewhat crippled by measures used by Google's YouTube website, making it unable to fully download for offline storage. It seems that this also extends to actual text within the page itself, since that is capable of changing over time (theoretically, once you archive a page you should not lose access to it even if the original website goes down).
We can only presume what the message means, as it is up to Google to make web pages that work with standard display tools. Perhaps they intend to stop people from using WebArchive in the future, or perhaps the message is a mistake or an oversight. Ultimately if you are concerned about it, you should contact Google for a statement of their intent, and ensure that you use Safari to demonstrate that the problem impacts all Mac software that makes use of this mechanism.
For pages that force content to be displayed dynamically, it is typically best to use the Project or Document Bookmark systems to store these links (refer to §10.3 in the user manual PDF, for further information on this feature). The preview pane in the inspector may still not display the page properly, but it will be easy to load the page into a fully capable Web browser by double-clicking the bookmark.
There may be other methods of more securely archiving content for safekeeping, but we encourage you to ensure that such services are in compliance with Google's terms of service.