Identifying legacy packages

[nia <nia%netbsd.org@localhost>]

Can we start clearly labelling why legacy packages are being kept
in their DESCR or by some other mechanism?

e.g. 

graphics/opencv2 - ??? nothing depends on this
graphics/MesaLib7 - Legacy platform support?
multimedia/libmp4v2 - KDE4 ... probably?
www/firefox52 - Last version without rust dependency
chat/inspircd - Still supported, chat/inspircd3 is semi-incompatible

In general, this helps out developers 10 years down the line who are
staring at these duplicates of up-to-date packages and wondering why
they exist, and prevents mistakes when including buildlinks.