I would argue they’re no more visible than the Catalyst. In all those cases you need to inspect and find stuff in the open game world.
I think it’s because they’re so old and core that you know about them because they’re from the time where you were leveling and hadn’t gotten so fixated on PvP yet. I honestly do.
If you can manage to repeatedly fly through Valdrakken and ignore literally every exclamation point, and by the way the main city has always been full of ultra important core quests, then… well, yeah.
This thing is not hard to find. All you have to do is inspect someone. Anyone. And then ask. And you did eventually. The game is doing enough as it is pointing you to everything with arrow. It’s okay to use the in-game chat to ask people. You don’t have to find anything on WoWHead - matter of fact they should kill WoWHead imo.
Your problem is you’re ignoring the world and you’re ignoring the people. This game is fundamentally designed around those two things as well as combat and a constant stream of content. Said another way, while you are probably very competent at WoW’s combat, you actually have no idea how to play the game overall - you’re actively ignoring 3/4ths of it, and tons of the last quarter of stuff you’re in to, and then being confused when something important was in all that lot that flew under your radar. Again, those 3/4ths are NOT raiding or M+. It’s Azeroth itself and its community. You’re ignoring them.
The Catalyst quest is straight next to one of the most important characters in the expansion, in the major city, it’s not a new system, and you can get hints to it by just thinking “Wait, where’d tier gear go and why is everybody else wearing it?”
So this is a very special form of “git gut” moment as far as I’m concerned. Hopefully you won’t find it insulting, it’s not meant to be. It’s just that WoW is something much different to the way you’re trying to play it right now and it’s actually punishing you for it.
EDIT: One thing I do not like, which is a punishment it gave you unfairly imo, is it doesn’t have catch-up. So now that you discovered it late you also can’t just fix it. You lost out. THAT sucks. But that’s not really a discoverability issue tbh, that’s a “FOMO” or fear of missing out issue.