I see that both linum and nlinum are actively maintained: one is built-in, another recently had a release.
With older Emacs version, linum used to be slow with a lot of folding in a buffer (e.g. in Org mode, and with hs-minor-mode in various source code modes). So I switched to nlinum.
Currently (on Emacs 15 and 26) I see that linum is sort of fast again, though maybe not always as smooth as nlinum.
Is there a reason why I should stick to one of them from maintainability POV? That is, does one of them obsolete, or is going to obsolete, another?