I think - and I hope it's come across in my posts - that I don't think it's ever acceptable to raise voices, threaten or be otherwise abusive to anyone in schools (or any other workplace!).
There are two issues, as far as I can see as both a parent and someone who isn't school staff but someone who works closely with schools and families every day. Well there are more than that - but if we're discussing parents getting irate/inappropriate/abusive/complaining.
You have the parents who get completely ridiculous and abuse school staff because their child can't have bright orange hair against school policy, or their kid has been a horrible little wotsit and has been rightly sanctioned, or the teacher has called home about homework and they don't like it. These parents are completely out of order and frankly I'd welcome steps to make sure their behaviour is stamped out in whichever way works most quickly and effectively.
Second set, still shouldn't be abusing anyone. However these are the parents who're at the end of their ropes banging their heads against brick walls, who have tried to address issues appropriately (whatever those are - SEN related or not. Could be bullying, could be anything!). They've spoken with teachers, they've spoken with HTs they've been calm, they've emailed, they've taken proper advice - sometimes lying for it - , they've had the meetings, and their issues have been going for months, if not years. They're either facing completely ineffective SLTs, or ones that actually just want them to go away now because they're too much bother and so they do their best to push them out. In lots of these cases, where things drag on and on or issues are repetitive the parents are right and the schools are wrong. Everyone assumes this doesn't happen. It categorically does.
Those parents and children - whilst still agreeing that they shouldn't be abusive to staff - are so heavily impacted by the schools actions or inactions across their entire lives that it's small wonder than in the end the get snappy and cross; or refer to Ofsted, or complain to Governors or the LA.
There needs to be action taken either way. No one should go to work and be abused, anywhere. But we cannot pretend that schools don't play a part in lots of cases in needling these families until they snap and get shirty.
I know of more than one parent - mother actually- who has been diagnosed with PSTD following their experience of advocating for their child in the education system. Actual PTSD. I don't doubt that there are teachers who've had the same, following their experience of working in a school. It's horrendous that it gets to that for anybody.