In my case, the “Split top” option disappeared when I updated /e/OS from 1.13-s-*-stable-FP4 to 1.14-s-*-stable-FP4.
You can also force “Split top” option for all apps checking “Force activities to be resizeable” in the “Developer options” screen.
I don’t know if it’s the same problem for everyone here (tell us), but I found the reason why the split screen option disappeared on my phones (FP3 and FP4).
The reason is that I chose to remove animations in the accessibility menu (-> Text and display)
I can’t understand the connection, but the fact is that this disables the possibility to split the screen. And of course, this possibility is back when I enable animations.
On a third phone (Moto G5, I can tell you the /e/ OS version later), the behabior is correct : no aminations, but still able to split the screen.
This makes me say that this problem is a 
real software bug .
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I had tried this, didn’t change anything.
Anyways, the split screen function returned with recent updates by itself. 
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