Ok, so...a lot of the research is US based... The discordant sibling study:
www.ncbi.nlm.nih.gov/pmc/articles/PMC4077166/
"Our results suggest that much of the beneficial long-term effects typically attributed to breastfeeding, per se, may primarily be due to selection pressures into infant feeding practices along key demographic characteristics such as race and socioeconomic status."
This is looking at the ethics of breastfeeding awareness campaigns, but also looks at the evidence used to promote it:
jhppl.dukejournals.org/content/32/4/595.full.pdf+html
Finally this article, which looks at research as a whole
sciencebasedmedicine.org/are-the-benefits-of-breastfeeding-oversold/
And sadly yes, babies are dying in the UK and USA due to starving to death from insufficient breastmilk, they are also at increased risk of severe brain injury, developmental problems and other unpleasant things. This is why Fed Is Best as a movement began, and they have stories as well as research evidence to back up their recommendations.
fedisbest.org/about/
I will apologise for saying that breastfeeding is not important, that's not what I meant and I posted in a rush. Breastfeeding is and always will be of importance, and to those people who wish to breastfeed and are able it is very important. Also to premature babies breastmilk is much much more important as a 'medicine' as well as a food. However, for full term babies I don't think it is as important as it is being made out to be, especially in things like the Baby Friendly Hospital Initiative (which in and of itself is causing a lot of problems globally).
The research into the infant microbiome is really cool, and new, and exciting. But as it currently stands, it's basically impossible to tell a previously breastfed child/adult from a formula fed one, when you remove other confounding variables.
When I said BF isn't important, I meant in the context of sharing the child care, someone (sorry not sure who) said that sharing the care disrupts breastfeeding (or can do) which is fine, but that also suggests that BFing is objectively more important than sharing the child care...which it is not.