But that's the thing, I've got plenty of apps... I would love that majority of apps I download support it. But if I had to choose.
Google Drive
Dropbox
Microsoft Office Apps
Google Docs & Slides
Youtube (still no PIP, I doubt it ever supports the feature)
Chrome (Albeit, I use it sparingly... I'm on Testflight, Darkmode looks awesome!)
GoodReader
FileBrowser
Just to name a few.
I’d expect the Microsoft apps to get support for it soon, since they we’re shown as part of the multi-window demonstration back at WWDC.
Google on the other hand I wouldn’t expect support anytime soon. Their office suite took quite a while before it ever supported split view, heck Gmail and Hangouts still don’t support it. Add the fact that docs/sheets/slides apps on iPad are hot garbage anyway, and I suggest uninstalling them and using the web apps now that Safari has much better support for them.
As for the rest, it should just be a matter of time. Implementing multi-window isn’t trivial, and with how rocky the betas were this summer I suspect some devs decided to wait until things settled down a bit before tackling it. If there’s one you’re really looking for, try messaging the developer and asking. I’m on a few testflight betas just from asking a dev a question that turned out to be in the next release, so they put me on their beta to help test it out.