If it has an online component Dark Souls, no absolutely not as that would divide the user base and ruin the premise of the game. For a game like Sekiro, it is not quite as black and white. On one hand, it is an offline game and would have no impact on anyone else. On the other hand, it may affect it from a design perspective and alienate the core fans, even with harder difficulties. The challenge would be less about how the game is designed and more about the stereotypical, "enemies hit harder and have more health" way of things. This is because the design would now have to take into account this easier mode.
So I say overall for both instances, no. The game is designed to be challenging, and if you do not like that, play something else. There are plenty of other games out there.