Everything is a red flag these days 🤗.
But in all seriousness the biggest red flag, perhaps the only one is someone who's not willing to communicate, realize their faults, and shortcomings, and engage with their partner to develop healthier behaviors in a relationship, or maybe that's a red flag due to unconfidence, and controlling behavior.
People are weird, hopefully people figure out a healthy dynamic for themselves, and find a relationship that works for them 🤷♂️.