From Markus Scherer's comments on Unicode support in ECMAScript
edition 4 (applicable to all editions from ES3 to ES6 draft 8):
Chapter 7.3 defines a small number of characters as LineTerminator characters.
Proposal: To add the ISO Control U+0085 Next Line (NEL) to the list of LineTerminator characters.
ECMAScript already lists several Unicode and ISO 6429 controls that are commonly used as line break characters, except for U+0085 Next Line. See <a href="http://www.unicode.org/reports/tr13/">UAX #13: Unicode Newline Guidelines</a> and <a href="http://www.w3.org/TR/xml11/">XML 1.1</a>.
(The additional comment on StrWhiteSpaceChar has been addressed in ES5.)
NEL was originally going to be incorporated into ES5 and was included in draft specifications through June 2009. At the May 2009 TC39 meeting (see http://wiki.ecmascript.org/lib/exe/fetch.php?id=es3.1%3Aes3.1_proposal_working_draft&cache=cache&media=es3.1:es5_candidate_errata_june_17.pdf item 7.2) it was decided not incorporate it as a line terminator or whitespace character. That decision was based upon a es-discuss thread that starts at https://mail.mozilla.org/pipermail/es5-discuss/2009-May/002676.html (<NEL> Considered Harmful.
Before adding it back into ES6 we would have to carefully reconsider the arguments that were made at that time and convince TC39 that they are no longer valid.
Thanks for the pointers – I wasn't aware of that older discussion, and agree with its conclusion.