According to reliable industry insider Tom Henderson, there is a separate Call of Duty zombies project in early development, which is not connected to any Call of Duty game.
Zombies is certainly one of the most popular game modes in Call of Duty games and with the latest instalment in Black Ops Cold War, the popularity is higher than ever before. Of course, it would be surprising if Activision were not looking to expand this universe in future games or even a completely standalone Call of Duty Zombies game, which is something that could be happening according to reliable industry insider Tom Henderson.
In one of his latest tweets, Henderson states that there is a Call of Duty Zombies game in development but this title is not connected to any previous Call of Duty game.
"There is a separate Call of Duty zombies project in early development, that is currently not connected to any title."
At the moment, there is nothing official from Activision regarding this so take it with a grain of salt, just like any other rumour or unconfirmed info.
It is, of course, entirely possible that Henderson's info is legit and such a game is coming. With the success of Call of Duty Warzone, another standalone free-to-play title, an independent Zombies game does not seem unrealistic at all.
All in all, these are some exciting rumours and hopefully, we'll get an official announcement and more details in the near future.