Advice is often more valuable when it comes from people less likely to give it. So maybe a good social norm would be something like: "from each person, one parenting advice per month". People who say little would be encouraged to speak up, people who already say too much would be reminded that they are over the limit.
Recently I made a post in our kids group about something Lily (9y) had done that was funny, mischievous, and also potentially embarrassing. A friend asked whether Lily knew I was writing about her antics and said they would have felt mortified and a bit betrayed if this had happened to them at this age.
I think it was really good they asked! While Lily knows I post this sort of thing in the group, and this time already knew I'd posted this one (and thought it was funny), the friend didn't know this.
Kids are in an awkward and vulnerable position, raised by people with so much easily abused authority, and I'm happy to talk with friends who think I might be being unfair to my kids.
I also wish raising this kind of concern were more acceptable in general. The friend phrased their question in a softened and guarded way and apologized in case it seemed prying, which I do think that was a reasonable choice given the chances it would be poorly received. This raises the cost of communicating anything, since it's more work to phrase acceptably, and even if ideally phrased some people will still take offense. Part of my motivation for this post is to make it clear that I'm open to this kind of feedback, and perhaps encourage others who are to let their friends know that.
Note that I'm not saying that society's bar for unsolicited parenting advice is too low: I think people are often too free to offer advice without some signal that it's wanted, and while receiving unsolicited advice rarely bothers me, many people really don't like it. Instead, it's specifically around noticing that someone may be being unfair to their child where I'd love to see society move a bit in the direction of friends speaking up, and parents taking it well when they do.