here’s the deal - no, whatsapp doesn’t notify screenshots. not for regular chats, not for view once, not for anything. you are totally safe.
i’ve been screenshotting stuff on whatsapp for years and nobody’s ever known about it unless i told them. view once messages are just messages that delete themselves, but they don’t stop you from screenshotting while they are visible.
if you want to save view once content just be quick about it. open the message, screenshot immediately, done. the sender will see you opened it but won’t know you saved it.