You can find it here:
https://docs.microsoft.com/en-us/answers/search.html?c=&includeChildren=&f=&type=question+OR+idea+OR+kbentry+OR+answer+OR+topic+OR+user&redirect=search%2Fsearch&sort=relevance&q=vsto
They're answering questions about VSTO addins as well so pretty useful hopefully!
English-speaking TechNet forums migrated to Microsoft Q&A forums in June 2020.
https://social.technet.microsoft.com/Forums/office/en-US/home?forum=visiogeneral
Russian speaking TechNet forums still work.
But Stack Overflow has:
https://stackoverflow.com/questions/tagged/vsto
and
https://stackoverflow.com/questions/tagged/.net
Far higher quality of question and answer.