The slide over and side view still exists, but how you get to to them is differnt.
Instead of using the gesture to get a app list from the right edge of the screen, you’ll be using the new dock. The dock can now be pulled up from the bottom of the screen. Just a flick will do it. Swiping up further gets you the Control Screen and app switcher combined view.
To get slide over, just drag an app from the dock onto whatever app is active. To use side view, drag the app to the left or right edge of the screen. The open app with scoosh over so you can drop the new app next to it.
The dock will now hold more apps than it used to. The icons get smaller the more you add. How many you can add depends on what model of iPad you are using. The right three icons are for recently used apps and continuity (unless you turn that feature off).
Pairing an app that is not in the dock is a bit more complicated. From the Home screen tap and hold the app you want to use in slide over (or side view), when it lifts from the background drag is slightly out of possition. Without releasing that app, tap the main app you wish to open. When the app opens, continue draggin the first app until it is where you want it (slide over or side view).
Multitasking is far more capable than before, but it’s also become a bit more complicated. It may take a bit to get used to, but once you do it makes a lot of new workflows possible. For instance, if you’ve got an iPad Pro you can have a slide over and side view app active at the same time; giving you three active apps on screen.
Here is Apple’s support page on iOS 11 multitasking:
Use Multitasking on your iPad