Yes, obviously the Helmut Singer GmbH is closed and since 15th January 2018 'in liquidation'. Search 'Handelsregister Aachen Singer', HRB 15785.
To finally close a GmbH, it's necessary for legal reasons, to 'liquidate' for 1 year, to fulfill possible claims from creditors, so that will happen in January 2019.
There had been some rumors already on microcontroller.net, that he wants to retire, but could not find a successor for the company.
Helmut Singer should be far in his 70ties already. (There's an 8 years old photo from elektor).
I have bought many RF tubes for our RWTH Aachen physics institute about 30 years ago, at his shop, where I met him personally... he's a really fancy person..
A pity, as several 'from stack' devices really were cheap and did indeed work fine, or with slight repairs, like the Fluke 5200A, 1979 vintage, for 225€ incl. shipment.
He had mostly VERY old stuff, and when being 'tested' they were way too expensive.
Frank