Does GTA 6 have auto-aim?

Yes. Auto-aim isn't gone, it's weaker by default. The claim comes from a forum thread titled "No more auto-aim, confirmed by TGG", but TGG's own preview report doesn't say that. Rob Nelson said the default has less lock-on than previous GTA games.

The aim still sticks to targets, and you can change how much in the settings. Rockstar also told IGN you can aim for arms and legs and disarm people instead of killing them, which mostly works when you're aiming yourself.

So on day one, open the settings and set aim assist the way you want before your first gunfight.