There is few information about NEL. I think it was supposed to replace LF and CR LF, but it seems like it wasn't very used.
Is it somehow deprecated and should applications interpret it as a new line?
There are many different code points which can be used as line separators, mostly for legacy reasons. Unicode's design seeks to preserve all information, so text can be roundtrip en- and decoded to/from Unicode without information loss. NEL is/was used in EBCDIC to represent newlines, so it made its way into Unicode as a separate character.
It's not "deprecated" in the sense that it still fulfils the function it was designed for, but you will hardly find it in actual use unless you're dealing with legacy EBCDIC somehow. If you do, you may want to treat it as newline, but many modern systems to date just treat it as whitespace.
The Wikipedia article on NEL has more information.