As I understand it is not supported at all. It does not mean that it will break automatically (as in the ruby-rails-2.3 example you gave) but we can not maintain it in a good way. One of the main reason for it not being supported is that the later versions do not even build on wheezy and hence it is hard to maintain. There could be many reasons for a software to no longer work. Environment changes is one example (as this nss update), but there are other cases too, like that the service the software uses has upgraded API or similar.
I'll make a new try to find out what the reason for this problem really is, mainly in order to understand whether this can impact some other application or not. But I will not spend too much time on it unless more people from the LTS team think this (chrome not usable) is a serious problem that must be fixed.
As I understand it, this is the situation:
- After the NSS upgrade chromium no longer work.
- Before the NSS upgrade chromium do work fine.
- Chromium has no Debian LTS support.
- Latest version of chrome can not install on wheezy (requires libstdc++6 (>= 4.8.0))
My suggestion is to upgrade to current Debian stable release, but there may of course be things that prevent you from doing that.
An alternative is that you downgrade your libnss3 package and "hold" it so it is not upgraded next time.