I think I know what the big taboo is about. It's to do with a mother's love for her child and how that is used to trap and silence her into a life of drudgery and caring - a type of silent martyrdom (you must nevert tell anyone, especially your children if you regret becoming a mother) so that men can get on with more interesting things.
Most of us think the nuclear family is the natural way for humans to live, but any evolutionary biologist will tell you that humans engage in alloparenting - shared parenting and that's how our brains are wired. We are certainly not wired for doing it solo or even as a couple. We are also not wired for teenagers living at home because they would have been fending for themselves in our evolutionary history.
The gap between our brains and our culture is where the taboo lies. Our culture (and men telling women what they are) uses motherhood to constrain women in a cruel way. We were never meant to give up ourselves, ignore our instinctual boundaries (e.g getting touched out, no time for self, child centered lives), but we are told it's a natural part of it.
On becoming mothers many women realise they have given up their agency and are understandably unhappy about it and it can feel like a prison sentence. But then we are shamed and told to keep quiet about it (cos you don't really matter any more).
As Susan Jeffers said in her book I'm Okay, You're a Brat, it is in the long term cruel and a bit vindictive NOT to warn your children how difficult being a mother is as you might lay the ground for them being equally trapped and unhappy down the line - not the same as saying you regret them. You frame it as a problem of society and a lack of support, not because of individual children.