When a song (or any content for that matter) has been deleted from Spotify probably because the person uploading it has decided to remove it. That song is unavailable and is Greyed-out in the Playlists. We (the users) have difficulty finding similar song replacements and sometimes there isn't any, however should something turn up later there is also no means of knowing it has.
So I suggest:
When it's Greyed-out, the system to mark it as Content no-longer exists also update-date it to when Greyed-out was first detected. Then provides 2 options to Greyed-out songs;
Search for similar content - this should initially be by fuzzy match of title and later better still a fuzzy match of content.
Place content on Watch-list this should place a Watch-list marker on the song (or any content) as being on the Watch-list.
A means to just list Watch-list items to be provided to make things manageable.
An automated process to analyse the Watch-list about once a month and to send out a notification of Watch-list items to the user showing the number of similar content items found, probably best to limit that to 10 with a note if More than ten found. No need to list anything found as the means made in Suggestion 1a above exists.
For the purpose of neatness within Spotify, it would probably be a good idea to automatically delete any Watch-list item older than a year warning within any notification message of any songs (etc.) to be deleted on the 13th month.
This may sound a complicated idea, most of what's needed to deliver it seems to already be part of Spotify.
Your idea has been submitted a while ago but unfortunately hasn't gathered enough kudos (100 per year). In order to keep an overview of the active and recent ideas in this forum, we will have to close this idea for now. However, this does not mean that your idea has been rejected by Spotify.
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