Unable to insert video into the watch later (WL) playlist

Question

I have a Google Apps script that's been running for a while without error.

It takes new RSS feed items from my RSS provider, gets some basic information about the video from YoutTbe and then adds the video to a certain playlist based on that.

Since a couple of days ago, while the rest of the playlists are working fine, adding any video to the WL (watch later) playlist results in the following error:

GoogleJsonResponseException: API call to youtube.playlistItems.insert failed with error: The API does not support the ability to insert videos into the specified playlist.

This is the request I am sending (again, working for two other playlists created by me):

{
    snippet: {
        playlistId: 'WL',
        resourceId: {
            videoId: '<videoId>',
            kind: 'youtube#video'
        }
    }
}

and it's being used like this:

YouTube.PlaylistItems.insert(resource, "snippet");

Additionally, I do log the contents of the WL playlist in the beginning of the script and the playlist items comes back as an empty list (although the list has videos in it). As far as I can tell, those two issues started at the same time.

Answer 1

According to the entry from September 9, 2020 of the official revision history of the YouTube Data API, the watch later playlist is deprecated w.r.t to using it with the PlaylistItems.insert API endpoint:

[...] The playlistItems.insert method's and playlistItems.delete method's support for these playlists are also now fully deprecated [...]

The entry from January 28, 2021 states that:

[...] While users' watch history and watch later lists are, indeed, not accessible via the API [...]

Therefore, you'll have to acknowledge that your use case of PlaylistItems.insert endpoint works no more.

That's unfortunate, but a (documented) fact.