NEWS.26 says:

'linum-mode' and all similar packages are henceforth becoming obsolete.
Users and developers are encouraged to switch to this new feature
[i.e. display-line-numbers-mode] instead.

This patch updates the FAQ to recommend display-line-numbers-mode over
linum-mode.  It also changes one defcustom in progmodes in the same
way.

WDYT?  And, if it looks good, should it go to the master branch or emacs-26?

Thanks,
Stefan Kangas

PS. I'm not sure if linum.el should also be moved into lisp/obsolete/
or if that's premature for 27.1.