Is there really anything true that we simply cannot say?
...
what highly probable or effectively certain truths are genuinely taboo?

1. Edolo sat ignore quidnis memere qui vertorum fugit.

2. A caret tibusdam sit altus docet: nonamus quor ultus exeati nec mensus essit biscripta.

3. In hero me digno neque abuntum et cupitersum obos caris femina vitabique? Prae? Tetimeo Dausa iam aud longistio ventis!

4. Hic volunt quod absenere linera dat revertas.

Or to put that another way, if there were, then by definition we could not say it.

I've found that the most frequently given examples don't hold water.

Well, if they're given, in public, they wouldn't be taboo. What are you really asking here?