When I think of a Tsundere, the first that comes to mind is Tsugumi, from Ever 17.
That person hated and disgusted by people, and that included you. At hardly any point during the game did we get the impression that she had any interest in us, except as we closer and closer to the end. After sometime she stopped being outright rude, but she wasn't nice to us either. By the end, we learn that she would've liked attention from other people as much as anyone, but she learned to be distrustful through experience.
Nowadays, most tsunderes make it quite obvious that they harbour feelings toward us, and simply refuse to be honest. That's still as much a Tsundere as Tsugumi was, but certainly a different kind.
What I feel makes a tsundere an enjoyable character, is how much more effort they require to have them trust us, and fall in love. As Katatsumuri put it, defrosting the "Ice Queen" is enjoyable.
With the new kind, called 'modern' tsundere, this simply doesn't happen, it's not that they don't like us, it's not that they have no emotions, they just refuse to admit it. Since I don't find these particularly enjoyable, I can't really tell what their appeal is.
Each of them is a completely valid Tsundere, and fit the definition of 'dere' and 'tsun', but in different way.
I'm not so well versed in anime and visual novel history to be able to comment on this, but I believe that in the past, classic tsunderes were more popular, whereas now, the modern tsunderes are more popular.