I think the idea is that starting at 58 will allow them to ignore any class-specific stuff in Legion, since that is all relegated to 1-50 leveling content.
Shadowlands, on the other hand, I have no idea how that will work unless they plan to sunset those abilities with the end of the expansion.
I imagine that the most we’ll see is that they might integrate some kind of NPC standing around in Dalaran that allows them to functionally bypass the breadcrumbs that would normally lead to your artifact/order hall questline. Like it just immediately brings up the mission table map where you are supposed to choose which zone you want to start in and sends you off to start questing for people who might want to collect some of the leveling transmog sets.
In the past, they’ve never gone back and added things for the past classes that were added later. I really do not think we should expect any artifact weapon or azerite armor, or anything to be added for the Evoker Class. They’re a hero class that will be 60 by the end of their starting zone, bypassing all these previous systems.
Some of them will probably wind up with the new talent system, at least the damaging/utility ability for your spec. We saw in the mockup for the Druid Talents, Convoke was there. Obviously none of that is set in stone yet, it’s a work in progress. The ‘For Fun’ abilities are probably going to be out but I’d like to see them stay.