Yeah it’s a shame that support is slow to be rolled out but that’s entirely upon the individual app developers and not Apple. Split view has been around for a while now, supporting it in their apps is just not as high a priority for some developers as other improvements they feel they need to make in their apps.
I’m still a Google Hangouts holdout for example but since that service is set to eventually be killed off I don’t anticipate split view to be supported in it. Some apps like this simply don’t support either mode (split or slideover) so there’s no way to force it into slideover mode.
I wish there was, just as I wish there was a way to force iPhone only apps like Instagram into some kind of windowed mode in landscape orientation on iPads rather than being forced to use a monster scaled iPhone app in portrait . . but that’s an entirely different issue/can of worms.