Since this is a user-to-user forum, nobody here really knows.
The Adobe staff that comes here may know, but are not allowed to tell us.
If you add your vote and detailed opinion to this topic in the official Adobe feedback forum, Lightroom: Can't use tethering with Nikon and Leica after installing MacOS 10.11 El Capitan, you'll be notified by email when there are any changes to the status of the issue. Adobe does participate in that forum, and Adobe Chief Customer Advocate Jeffrey Tranberry has replied in that topic.
is it ever going to be possible to upgrade to another OS for Apple computers?
That's really up to Nikon, not Apple or Adobe -- Nikon provides the software libraries for tethering their cameras. By their own admission, they failed to test the libraries and their own apps on El Capitan before its public release, and they have not provided any updates as to when or if they will release new versions.