the feature hasn't aged well
I guess to some extent this is the big problem with a lot of these features. They took a lot of work to develop, they were very useful and interesting additions to the game originally (YEARS ago) but now they're outdated and close to useless because nobody at Sony ever bothered to look at them again and ramp them up a bit to make them currently relevant. To then expect new and returning players to learn about them is not only close to useless for them but also makes VERY clear to them that a lot of aspects of EQ are out of date. This discouragingly reminds them that they're returning, or starting to play, a very old and somewhat creaky MMORPG.
As a veteran player I love the esoterica and mystery found in the depths of this game. But I think the same things can be offputting to new players, especially if there's little benefit beyond nostalgia in learning about this stuff.
I won't deny this kind of depth can be attractive to some gamers. But NOT to most. Unfortunately, if EQ wants to draw new blood the game has to at least have the potential to be attractive to most gamers. Unless Sony's "game plan" is to maintain EQ "as is" and wind it down over the next few years while putting most of their developmental effort into EQ Next.
This is no different from any other game around EQ's age. If you tried to play Asheron's Call for the first time now, you'd drown in all the stuff you need to know. Same with Anarchy Online (What does 'OE' mean?) and to a lesser extent, DAoC. Heck, EVE is younger than EQ.
The only way to avoid that is to wipe and relaunch the game every 2 years. Any game with a decade or so of content is going to have dozens of systems that weren't further iterated on. New players will find a game that suits their playstyle and desire. Some people want a mindless experience where the game basically drags you to max level, kicking and screaming, whether you like it or not. Other people want a game with depth and history and stuff to do. Some people want a little bit of both extremes. Right now, the amount of things you need to know to function in EQ is really small. Like you allude to, many features have fallen into the abyss of obsolescence so knowing about them is optional (shrouds, for example. PoP flags for another).
Basically, where we are in EQ is the result of EQ's longevity. I don't think that's such a bad thing though.