I am describing extended features that may accompany this feature, and can be posted in a separate thread, and are not necessary for this feature in order to improve the status quo.
If an observation has a dead video URL (or the video has been removed ), and no photos, screenshots of the video or audio is uploaded, and the URL is not backed up by iNat or made available otherwise (e.g. via a Wayback Archive link or a link to it stored and publicly available elsewhere) then the community can flag the observation as being no longer valid.
If the video is not backed up or screenshots aren’t provided by the author, then not only the author, but others users (particularly if the video has a permissive license,* although the default for YouTube is unfortunately all rights reserved), could also take screenshots of the video, save them elsewhere that’s publicly shareable (e.g. IPFS, Imgur, cloud storage, etc.) and post the link. * Note that even if the video has an all rights reserved copyright license, I can’t imagine it would be illegal to take screenshots from a publicly listed video, share them publicly, and link back to the source. For this scenario where there is a video, it may be helpful to also allow other users to upload photos and audio that are extracted from that observation. Moderators and other users can flag content that does not appear to be from the video, to prevent abuse of this feature.
The same feature would be useful for audio observations, for example where original audio content is uploaded, but the author hasn’t edited it, and the edited version may (or may not) help with trimmed silence, adjusting the gain, and filtering. Same thing for photos, e.g. where a photo may show an ant or something small or hard to see, and the image could be cropped or the specimen could be circled to see it clearly. While it may be preferable for authors to edit content before uploading, that may not happen, so providing the ability for other users to be the editors and then other users to moderate can help.
Get creative, think outside the box!