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?
display-line-numbers-modein recent Emacs versions. It uses code implemented in C, so it might be faster than both, and it ships with Emacs, so there will be an incentive to maintain it.