If you read the post above from an Epic employee, the support for Metal was not complete OSX 10.11 support is a lot of work for them to do, so they have a lot of work to get it compatible and don’t want to waste time when it’s still in beta.
It’s unlikely that UE4 will support OSX 10.11 the day that it launches, given that the updates to UE4 don’t line up with those types of releases, for example Windows 10 launches this week, but it will be a while still before we get UE4 4.9
Though given how many people update OSX when a new version is out, they may prioritize having support available if UE4 doesn’t work on it at all.