Let me share with you what I'm thinking of when I talk about 'true' subs, as I understand it's a broad statement. I can offer a perspective that is more nuanced, if longer to read. I'll bold the key statements.
I understand that when subs get large enough, groupthink emerges; people voting up/down not based on whether a comment contributes meaningfully, but whether or not they agree or feel good about it. Thus even constructive minority voices are drowned out.
The reason the splintered subs could be a problem is that it often left the disruptive people to represent entire ideas, for better or for worse. It fractures movements that should otherwise have common goals into smaller and smaller slices that are unwilling to co-operate towards otherwise shared goals.
The one that comes to mind for me is r/childfree. It started out as a resource for those who'd chosen a child-free life to find support, collate a list of recommended doctors that recognised body autonomy (its often very difficult to get sterilised, especially if you're younger and/or don't already have several children), how the workforce treated them differently for being child-free (such as expecting them to cancel their plans and sacrifice their time off for parents on short notice), impact on their social lives, etc.'
However, over time it stopped being pro childfree lifestyle choices, and support for a group that is often seen as 'selfish'; and started becoming anti child lifestyle choices. The frontpage became mostly rants, filled with terms like 'crotchfruit', 'breeder', etc. What was once a community of a minority lifestyle trying to find support and legitimacy gave way to anger and tribalism.
Eventually enough of the users that consider choosing to have children to be an equally valid lifestyle choice - merely one they'd chosen not to live - slowly started lurking, unsubbing, or otherwise becoming invisible. Anti-child/'breeder' rhetoric became more and more prevalent. Eventually, r/truechildfree was founded to do what r/childfree used to - collate resources and support for those who have chosen a child-free life in a world where children are considered 'opt out'. Thus childfree users split into pro-child and anti-child tribes.
Which is lovely for r/truechildfree and its users (I am child-free, but I like children; I just recognise I am not equipped to raise them). But it meant that the largest and most visible sub, r/childfree, became almost only child-haters, and an already maligned community often considered 'selfish' is now represented by absolutists that are no longer willing to respect people who disagree.
I understand that it is the nature of humanity, once pushed, to push back. I understand why those who see mistreatment in the workplace or socially for their choice to be child free would be upset, same as anything we hold close to our hearts. That pain is why the r/childfree support group came to exist in the first place.
But it is diversity of opinion that makes discussion so interesting, that allows us opportunity for growth, that has us looking at the ways we are similar instead of fighting over the ways we are different.
I think the anger of those in the new r/childfree is real, valid, legitimate.
I think the users of r/truechildfree's discomfort with how that anger was displayed is also real, valid, legitimate.
I wish we'd looked for a better way to handle it than for letting communities devolve into absolutism, though. Whatever your reasons for not choosing to have children, you still deal with the same stigma; it's a shame to have people who are struggling against the same chains to schism over the metal they're made of.