Interesting, thanks! For Austria this refers to § 61 Nationalbankgesetz (law on the National Bank):
https://www.ris.bka.gv.at/GeltendeFassung.wxe?Abfrage=Bundes... (in German, unsurprisingly).
That reads (my rough translation): (1) [The ECB and other central banks emit Euro bank notes.] (2) The bank notes referred to in (1) must be accepted at their full face value without limits, unless the obligation must be paid using some other means of payment.
IANAL, but to me this means like (a) there is already a law in Austria obliging businesses to always accept cash, except that (b) this might be meaningless since a business could always stipulate that "the obligation must be paid using some other means of payment".
