I think they are called classic layouts because in the original emulators there was no ability to have graphics, other than reels. So when the ability came to implement graphics, the term 'DX' (I assume shorthand for Deluxe?) came about, and the older looking, non graphical layouts became 'Classic'.
But simply, a classic layouts is any layout which doesn't have graphical lamps, normally doesn't have the actual machine artwork. But it has all the bits working - and that's part of the challenge.
I've made classic layouts, I've made DX layouts from scratch, and I've made DX layouts using an existing classic.
The hardest in terms of logic is a classic layout, or a 'from scratch' DX. You are starting with a complete blank canvas, and need to work out the machine inputs (switches, opto sensors, coin validation) and outputs (reel positions, lamps, and probably hoppers, tubes, and meters too).
On some machines it's almost like 'layout by numbers', especially if the machine has a test mode - once you've found your way into it - that gives you reel symbol positions and lamp positions.
On some it's genuinely a massive game of trial and error until you find the right numbers.
DX layouts based on classics, while they have a graphical challenge, have the machine's inner workings mapped out. So basically you start with an already working layout, and you then work with graphics to put on top of it.
I'd hope people who create classics would be recognised as equally but I'd expect that to most people who come across as a site like this, the classics look 'basic', so probably don't get appreciated as much.