I've seen that the plural was deleted back and forth. However, there can't be any discussion about it. It is quite easy to attest it even from the titles of professionally published books in google books (not to mention their content). And the scientific newspaper corpus at www.dwds.de has 16 hits in Berliner Zeitung (1994-2005), 15 in Tagesspiegel (1996-2005), and 79 in Die Zeit (1946-2018). Thus, it is not only a valid but actually rather a common form.
- In regular speech it's uncommon and surely deserve a note a like Elter. Duden too gives no plural and DWDS even states "wird nur im Singular verwendet" (is only used in the singular). -Der Jud (talk) 16:45, 24 August 2018 (UTC)