I've always used File > Import > Install > From Existing Installation to bring over plugins from old Eclipse releases. However, in Neon, it doesn't work when I select my Luna directory.
I used this process:
Error message says "Select at least one available element to be installed". There are no items listed in the list box area.
Is this a known issue in Neon? I don't see any relevant errors in eclipse-workspace/.metadata/.log
According to my Eclipse bug report, this is explicitly forbidden in Neon.
Source bug report: https://bugs.eclipse.org/bugs/show_bug.cgi?id=490515