Slide Over is gone, yes, but Split View is still there – a little longer in steps to reach it, but it’s actually more capable depending on your iPad size.
1. Put an app into windowed mode. I find the quickest way is to tap an empty area at the top of an app.
2. Long-press the traffic light icons. You don’t need to tap. You’ll get the same options as if you tapped and then long-pressed the green button.
View attachment 2554819
3. Under Fill & Arrange, tap the second or third icons for a two- or three-way split.
4. Repeat to add another app. The most recent app on which you tap one of those icons moves to the left.
View attachment 2554820
This can also be achieved by bringing two windows close to each other. They don’t have to fill the screen height or width. A handle will appear between two windows when their vertical edges are close together. Dragging a handle resizes the apps either side proportionally within their combined width.
View attachment 2554827
On 13-inch iPad, you can manually split the display between even more windows. Notice the handles between each pair.
View attachment 2554821
Of course, this leads to great practical possibilities for window management when you’ve got an external display and a mouse or trackpad connected.
Sadly, you don’t get handles between vertical splits, such as within the quarters view that’s available among the long-press options.