I don’t do it often. But I think it is normal to occasionally reference annoying shit we put up with in the past lol. It’s also a natural thing to discuss as those things eventually led to us being single :)
But to your point, if someone is venting about an ex when you just started dating them, that is a huge red flag to me. My S/O and I talk about anything and everything. This just occasionally comes up. It’s far from a 24/7 news cycle on shitty exes with us.