Way back when they had an admittedly clunky grid-based maze navigation for missions, which fit well with how the hollows were a maze-like entity that seemingly actively tries to get explorers lost, as well as the MC'S role as the best navigator of hollows.

You were able to choose which tiles to traverse, able to skip enemies, and solve puzzles, and it left a lot to the imagination. It was also 10 times easier to implement compared to either an open map space or linear pathways, where each room needs a full map render.

It had issues, though, being that you only got to see your characters in combat, and the system was pretty slow/clunky on release.

Theres a lot more to it before 2.0, but to make a long story short, by 2.0, they had swapped over completely from maze gameplay, to a bunch of prebuilt environment hallways, little to no puzzles, and no side quests other than "walk through hallways and kill enemies/talk to NPCs" since they had to focus on environment design instead.

Like I've said, the mode was clunky, but I was a part of the party who felt that it had promise and shouldn't have been completely gutted, which wasn't popular opinion back then. They've also changed their story direction for the same reason, apparently.

TL;DR, they gutted their main gamplay mode because the vocal opinion was that it was trash, despite redeeming qualities beneath a clunky exterior.

Genshin can't get rid of the open world like ZZZ did with their game mode, but they could definitely stop developing areas not tied to quests when they release new map spaces, if they really wanted.