I haven't seen a single real time diplo once warzone was introduced. May just be coincidence, but why are they so infrequent now? All the diplos are multiday now. Why did this happen?

Probably because it takes a long time for diplos to start, especially long ones. People don't have the time of patience to wait for 30 min to play a game lasting 3 hours where whoever is closest to booted players wins. Also makes it harder to plan alliances and strategies in real-time

Yeah, that's true. I just found it strange how it was abandoned so quickly.

It's also a question of when you're online. There's certain times where diplo players (on the whole) tend to be more active than at other times. Right now there's two RT diplos well on their way to being filled:

https://www.warzone.com/MultiPlayer?GameID=14838194
https://www.warzone.com/MultiPlayer?GameID=14837785

I think the Unity update may also have fragmented the diplo community slightly, but not by too much.

While I'm certainly new, only almost pulling in a full year playing, I can confidently say that every single real-time diplo I've played, whether it was right after I joined Warzone (Warlight at the time) or just a month ago, has been a bad experience just because of the rampant booting. Because of this I was turned off from them entirely, and I wouldn't be surprised if many others just got tired of giving them a chance not to just be a boot-fest.