If I can add a few words.
There is good reason why there is not a prescriptive list of content. The world is a varied and Technicolor place there are many different discussions we can have and many different ways of expressing ourselves.
A moderator has to use judgment as to the appropriateness of content. Not always for the content itself but the likely direction a thread may take.
This is never personal.
If I can illustrate a typical grey area - now please be aware I am making this up and am not referring to any specific thread - past, present or future (and any likeness to the living or dead is coincidental).
Example:
Post 1: School bans mobile phones.
Post 2-7: Discussion on where's and why fors.
Post 8: Side track onto religious objects (eg headscarves)
Post 9: Comment on PC brigade
Post 10: Comment on "britishness"
Post 11: Comment on asylum seekers
Post 12: Comment on get back where they came from...
Post 13: Comment on outrageousness of previous comment...
Post 14: All out cyber war between those who agree and those who don't.
Question, when is the appropriate place for me as a mod to split, edit, delete, remove?
We see quite a few posts degrade into argument - I'm even guilty of taking part on occasion. The prime focus is for RC to be a pleasant place to be. No personal attacks, no hatred etc. Now for me personally I would split that thread at post 7 and post a note explaining why I'd locked the thread - because I would lock it.
Why would I lock it? Because I see no value in discussing my judgment publicly. I can never please everyone so I'm not willing to spend the precious minutes I have to help people debating.
Another mod may see it and say whoops no time to edit/split etc as I 'm flying through before going the laundry so I'll quarantine the whole thread, intending to review later what can be returned.
Am I just rambling now? Probably!

The example above only shows my judgment. Other individuals will have different opinions - other mods will have different opinions. That's why we discuss.
There has been a lot of chat recently regarding PMs from mods to explain an action. In the past I quarantined a thread with 15 different contributors - 15 PMs, x% of replies to read and respond to. Cor blimey, - bear in mind I'm writing this message in work after 14 hour shift. No lunch no dinner

POOR ME!

We do our best and will continue to (hopefully) keep our judgment on the side of peace and equality.
I will try to write a full guidance post but in my simple mind it is quite simple.
This is a family website we have young members.
Everything you post can be read publicly.
Think - is it appropriate for a child to read?
Can what I write be seen as inflammatory?
I am not going to provide a list of what can and can't be posted as that removes the posters judgment. Think of the principle and spirit of the website.
Family History ROCKS! Let's not forget that is why we want to be here.
I'm happy to discuss any of the points by PM in relation to specifics but I will not discuss another person's judgment. That would be unfair.
All the best,
Pam