Let's recap:
1. The proposal is to offer mothers a 12 months leave, at the very least as a social expectation, even better paid by a state run insurance, see e.g. unemployment, to take care of their infant children.
2. Mothers rather than fathers because mammals feed their infants with milk, and milk is produced only by mothers. Human mothers need to feed their infants every 2-3 hours. As long as breastfeeding takes place, simultaneously working a career is pretty much out of the question.
"The American Academy of Pediatrics recommends that infants be exclusively breastfed for about the first 6 months with continued breastfeeding along with introducing appropriate complementary foods for 1 year or longer." https://www.cdc.gov/breastfeeding/faq/index.htm
3. This arrangement will create gap years and have a measurable effect on mothers careers. Specifically their wages will be smaller compared to same age fathers.
Working the chain of reasoning backwards, anyone that axiomatically insists no wage gap should ever be measured between men and women, sadly a very popular stance, is (willfully?) ignoring the motherhood aspect. Men and women are not fully interchangeable.
I don't see where the catastrophizing charge "excluding half of the human kind from the social sphere" comes from. As stated above, there is at least one critical area in life, specifically infant care, where men and women are not perfectly equal. Not being perfectly equal does not imply men and women must be maximally different to the point of "exclusion from the social sphere".
Until we drop the pretense that men and women are perfectly equal, we are going to produce inhuman outcomes. This is not even a new observation, the ancients had the story of Procustes holding people equal by the standard of his iron bed and fixing any deviation from his equal norm via blunt force.
https://en.wikipedia.org/wiki/Procrustes