I noticed while playing that every time a champion dies or a point is neutralized/captured the screen displays "-2 enemy nexus health" but nothing actually happens. Is this a bug or an unintended message?

Might be an intended feature that they forgot to turn the "on" switch for?

The team with the majority of the points are immune to Nexus damage, so I've heard. That might be it.

After a certain point, this no longer applies. It works at the start though, or is supposed to. You can't finish people off this way, but you can get ahead.

i personally think it's a bug.
i don't think i've ever seen it drop yet. likewise (i may not have been paying attention) when you capture a point or neutralize one, it takes life off too but haven't noticed it.
also, it would make no sense if only the winning team can hurt the opponent teams nexus if they kill them... because that would put the losers down even more, making it impossible to bounce back, though I personally have won despite being -200 than the other team at first

Actually is yes its -2 nexus health until nexus HP below some point, (100 health maybe) (there wont be -2 health). To proof this, you can see at beginning of your game, where 2 point capture it at the same time, both nexus health is 496 vs 496. There is another -2 health: your champs killed.

If your nexus is below some point, your revive time become shorter and shorter (depending with captured point too, if your numbers captured point is lower than enemy numbers in this condition, your revive time will be shorter)