Look, I get some things ending - like achievements. I mean, that’s the point of an achievement that is specifically denoted as “doing it before a deadline.”
I will go to my grave protesting removing items from the game. Whether it’s the AotC mount (and to be clear, I have them all), or challenge appearances.
It’s dumb. Plain and simple.
Mount drops should be whatever ungodly fraction of a percent drop mythic mounts become. Objects should still be attainable. Why are the belts going away? You didn’t get rid of the ring, so why remove the belt? Do you know how annoying it was to drop what I was doing to power 12 alts though overcharged delves just to make sure I had the silly things, just in case?
As for challenge rewards, again: the reward is a reflection of your capacity to do challenging current content, not that you had the good fortune to be playing (or alive, since some of this stuff was available before people were born). That’s not skill, that’s luck.
Challenge appearances from any expansion should be available to anyone who passes the current expansion’s challenge content. Let’s say M-15 (because I don’t recall what the current tuning is, and I unlocked the purple crescent BoP Scythe back in BfA on a M-15, but use whatever the current iteration of an “M-15” would be). You complete an M-15, you get a maguffin. You turn the maguffin in for a reward that is one of the challenge appearances that have been put in game. Maybe that’s killing the end boss in heroic or mythic raids (maybe mythic drops 2, maybe normal drops 1/4 of one, so after 4 kills you get one - I dunno, though I wold think Blizzard would be thrilled to have incentives to engage in all sorts of content in a way that isn’t always power). Then you just move the threshold every “season” and whammo, you have endless rewards to doing challenging stuff, and people remain endlessly engaged.
Seriously, it is incredibly foolish to remove stuff from the game, and easy enough to put all these things on a vendor you trot out every season, unlocked by various types of challenge content, to give people something to strive for. It’s not rocket science.