I just noticed that, while being logged in to Gmail (e.g. having a Gmail tab open), I am also automatically logged in to YouTube. This did not happen before. I used to have my Gmail open whilst navigating YouTube "anonymously". Suddenly, this seems to be no possible anymore. And, if I log out of YouTube, I am immediately logged out of Gmail.
Is this an issue specific to me? Or are you also experiencing this change? Has there been a change in Google rules recently about this? I see that the last change to the ToS happened on the 22nd of January of 2019. Yet, I just noticed this change this week.
Update: my YouTube account is actually empty (I have never used it, not even to comment). So I deleted it (it seems what I deleted was a "brand" associated with my Gmail account). Now I can navigate YouTube anonymously, as before. Still, this doesn't really answer the question of whether something changed. And, if this is a more general issue other people face, deleting the YouTube account is clearly an extreme solution.
Update 2: I use Firefox.