However, neither then or now does this mean a teacher has ever been "forbidden" from doing so, one way or the other

I am not a teacher, my opinion means nothing, and you obviously know better than I.

But.

A good friend of mine is a high school history teacher (Canada, so requires a 4 year bachelor plus a year of grad school and a shit load of other requirements) and he came under severe pressure to stop teaching kids about how Natives were treated in our country. Kids were going home and repeating true facts to parents and they felt that their kids were being bullied or some shit into feeling bad for being white (this man is severely white).

Was he forbidden from teaching this?

No.

But he came under scrutiny and he was advised repeatedly - off the clock and off school grounds - to stop focusing on residential schools in recent history (last residential school in Canada was closed in 1996).

The only reason he kept his job is the Truth & Reconciliation Committee) released their findings and that made it impossible for anyone to deny that that was actual history we need to pay attention to.

My mom was a teacher turned principal in a very bible belty school district and she was also never forbidden from allowing certain things (re: Harry Potter books in the library), but there were definitely consequences for it.

My rambling point: You teachers - especially American teachers that don't get the $80k+/yr that Canadian teachers do - have such a shit situation because I know many times you aren't forbidden from teaching something, but it will still affect your career if you do.