In the latest exclusive interview for Variety Hidetaka Miyazaki, the FromSoftware president, revealed information about the source of inspiration for Sekiro: Shadows Die Twice. Here is the full quote:
A lot of influence came from Tenchu. We even pondered upon making Sekiro: Shadows Die Twice a part of the Tenchu series at first. Of course, after that we researched and referenced many other games as well; however, I think the largest influence was from Tenchu.
This also confirms the words of certain employees of FromSoftware, who said that Sekiro eventually turned into a completely different game than it was supposed to be at the beginning of development.