The Feebas and Milotic puzzle in RSE was a genius mechanic built carefully around the lack of our current mass information source to reward the people who truly cared to play the third generation games for all their worth. Something has become a thing of the past.
You're given a few trainer battle encounters to help let you know the two new pokemon exist. You're given an AREA location of somewhere in route 119 for Feebas only. And that's it. And in 2003 the internet for most people (especially since most people playing were children or adolescents) wasn't a viable option to figure out how. It took a year or two for some people to stumble across Feebas, then a year or two more to stumble across evolving it.
Imagine the sense of shock and reward those lucky people got to experience thanks to that. But now, information like that is available almost immediately after games release and at any time you need it. And so games don't even try to put amazing secrets like those in the games anymore. And even if they did, most people would say it's unfair or cheat around it anyway. So then games had progressively less and less content like that after Gen 3.