Death Stranding was released last Autumn as the first game from Kojima Productions. Most assumed the team had started pre-production on at least one other game before Death Stranding was launched, but whatever it was - it seems like we'll never actually get to see it.
In a Japanese interview with Livedoor (translated by VGC), Kojima had this to say regarding his upcoming plans:
"It's still in the early planning stages so I can't say much, but on the down-low I'm actively working on it.
Recently a major project of mine got cancelled so I'm pretty pissed, but that's the games industry for you."
There have been rumours about bad blood between Kojima and Sony, but we don't know if the latter was involved in any way. Still, it shows that even one of the most famous people in the industry can't get green lights for everything.