For quite possibly the first time in my life...I'm gonna say True! >_<
I think ignorance can be a shield from some things-- the magnitude of blame and responsibility attached to an action, for one thing-- but in general it's a bad one (especially in this day and age), and in most cases, not one at all.
It definitely isn't and should never be a shield from critique, in particular. Critique (good critique...) is supposed to teach and explain; if you use your ignorance as an excuse to say you shouldn't have to listen to people telling you what you don't know, then ignorance clearly isn't your only problem.
But let's take this a step further: is anything a shield from critique?? Again, we're talking about real critique, not harassment (people tend to get them confused, often on purpose...let's not get fallacious here).
Off the top of my head, I can't really think of anything...besides maybe consent?
Like, if someone is engaging in truly harmful behavior that other people need to be warned about, I think you should probably call them out whether they want it or not. But if it's just something you personally don't care for, I think the person's right to be left alone supersedes your right to critique them.