Youtube pretty aggressively blocks automated usage now. If you connect via a VPN it won't show any videos until you log in. Considering that ad income and real engagement seems unchanged, it possibly is just that they started blocking bots better. Something you wouldn't strictly expect a public announcement over.

I also find that they block incognito mode a lot too - esp. on android when you use a hacked client like revanced (less so in the browser, but i use firefox and have seen incognito mode causing youtube to block you from viewing without logging in).