Since becoming a parent for the first time last year (DD is one next week) I’ve been shocked by how often parents are infantalised. Often told information that’s not really true just to make them feel better.
Things such as
‘fed is best’ - no fed is the bare minimum expected of a caregiver. Factually breast milk is better for your child than formula - and I say this as someone who chose to formula feed because I like my sleep 
This also spills into weaning, with parents who gave their 10 month old nothing but chips for dinner told ‘well he went to bed with a full tummy, remember fed is best’ no no no.
Food before one is just for fun - again a total myth, food before one is essential for oral development and nutrition as there are a fair few nutrients that aren’t passed well through breast milk or formula (Iron is the main one) but many doctors, health visitors etc. keep telling parents this to stop them worrying about their child’s eating, but it’s simply not true. It then leads to those parents telling others this myth making whole swathes of parents not consider food before one as important.
‘You can’t do anything to help kids meet their milestones faster’ - again, factually incorrect. There are many exercises that parents can do with their child to help them roll, crawl, walk, talk etc. a lot faster. Yes it’s not necessary to do those things and most children will hit their milestones eventually, but it doesn’t make it any less inaccurate that parents ‘can’t’ do anything to speed this process along.
Those are just the ones I’ve seen and heard over the last few months but there are many more I’ve forgotten along the way.
AIBU go get annoyed about new parents being treated like idiots, or is it maybe the case that many new parents are idiots and this kind of nonsense is necessary