Q: What happened to the split view with iOS 10 on iPad? Before the download I could open pages next to safari or many other apps.

It still works as before, or should.  Check your settings, general, multitasking and make sure it did not get turned off somehow.

Not on an 1st gen iPad Air.

Split view only works on both iPad Pro models, iPad Air 2 and iPad Mini 4.

Slide Over, which is the slide out panel (which isn't split screen view), works on iPad Air and later and iPad Mini 2 and later.

Hi. You might update the device in your Profile.