The TOS Enterprise was a Starship Class.
In my headcannon, Starfleet adapted a new ship classification system, during the TOS years. That led to the ship moving from "Starship Class" to the more specific "Constitution Class"
I've... got mixed feelings about where the show's going. I'm weird, I do not want Discovery to jump into the 28th century; I like it just where it is. Burnahm and Spock's relationship is something I'd like to see continue into and beyond his tenure in TOS, for example. I'd like Tilly to get her captaincy and rub shoulders with Kirk and co on another Constitution. I'd like more run-ins with TOS people and places. It's a nice segway into remembering Star Trek at the beginning, before TNG made us try so hard to forget it.
I don't want to "appease canon" when the show's done so much to make us love a new crew in this time and place.
Agreed. I like Discovery and I like the 23rd Century setting. I think it's been fun getting some back story to TOS and exploring those characters and the era more deeply. It had so much less attention than the 24th century or even the 22nd century got.
I'm enjoying the ride, but I have mixed feelings about a change in setting.