According to an update of 23 June 2021, published by the German Ministry of Finance, the mutual agreement between Germany and Switzerland, signed on 11 June 2020, regarding the taxation of income earned by frontier workers during the COVID-19 pandemic is prolonged until 30 September 2021. Previously, the mutual agreement had already been prolonged until 30 June 2021.
The text of the extension can be found here (in German).