What's left are shelves and the two wooden sides of the bookcase itself. The shelves can be ordered: https://www.ikea.com/us/en/p/billy-extra-shelf-white-1026529...
So I would rephrase your "no item numbers apart from connectors" to "everything can be replaced except for the two sides of the main body". And even those you can get by ordering a new cabinet without any doors or extras, since that's basically the sides + a number of shelfs.
This is clear because, as you say, you cannot buy the sides, or the unique bottom piece on its own. And the piece which is most likely to degrade is the one which has many connectors pushed into particle board potentially multiple times - the sides.
Maybe it was a poor example, but I think my point still stands. There are many other furniture products for which no spares are sold, or happen to be available to purchase for other reasons.
Coincidentally this appears to be changing as of yesterday - https://www.bbc.co.uk/news/business-55796429