Surely parents always knew what their child's teacher's name was? Even in the 70s or 80s?
Then they'd know, from other parents, or by having older children further up the school who teaches the next year etc So, there is an expectation of who will be teaching your child next year, assuming single form entry.
The school announces in newsletters who is leaving, and sometimes who is joining. When the newsletter doesn't say what the new arrangements are, you know that either the headteacher doesn't know themselves (worrying) or they know but realise the parents won't like it (even more worrying!).
Its not like there are hundreds of teachers, maybe just 7. So, it is not hard to count the couple of names who are leaving and divide by 7 (or 14 for two form entry) - hence the %.
The odd teacher coming or going for normal reasons e.g. promotion, retirement, relocation, maternity etc might be disappointing if you thought s/he would be good for your child. However, when a third of the teachers clear out at the end of each year for two or three years running, it does suggest that there may be another explanation.
If there is a problem with the school, and its making its way into the staffroom, then its likely the parents will know about it anyway because an overbearing HT (for example) is probably difficult with the parents as well as the teachers.
That is one big difference from my childhood - parents interact with the school much, much more. My parents simply got me to school on time, in the correct uniform and nagged me about homework. My parents never once had to ask the school to give me harder work (for example) but I've had many conversations with my DCs teachers trying to find a solution to the work being too easy for months on end - insufficient differentiation they call it - see i even know the lingo!