I followed the sources they link and the ones those link and found that the best substantiation they have is “according to the accounts collected by some NGOs”.

I don’t doubt there must be unspeakable shit happening, there’s been a war for so long that monsters are bound to take part. But I’ll hold my judgement as to how systematic it is until evidence is presented, not just claims by notorious liars who said the same shit about Hamas without any evidence and no pushback from these very same publications.