I was trying to build my setup scripts with Inno Setup these past two days (1 & 2 Jan 2021) and the digital signing was failing. So I contacted Verisign via email and await their response.
I received an official response from Verisign this afternoon (on 2 Jan 2021):
Thank you for contacting Verisign Support.
This server was deprecated after our authentication services were sold to Symantec, which is now Digicert. You can find a list of free timestamp servers online or theirs is now at http://timestamp.digicert.com.
If you have additional questions, please do not hesitate to contact us.
So the http://timestamp.verisign.com timestamp server is no more.
At the moment I know of the following alternatives (in addition to Digicert above) which work well:
http://timestamp.comodoca.com/authenticodehttp://timestamp.globalsign.com/scripts/timestamp.dllhttp://tsa.starfieldtech.com