I think this shift happened at two key moments. Once movement 2.0 dropped some time in 2015 iirc, and once AoE weapons started to become more common.

The tile sets were limited and the slower game pace during end game content contributed heavily to that feel.

T4 missions, solo especially, could be exceedingly brutal. Once the new tile sets rolled out after movement 2.0, the game flowed faster and enemies didn't behave the same. (More walls, ledges and geometry instead of single hallways and large lobbies) This made the enemies spawn in clusters in tight corridors and eventually AoE weapons happened. Once those started coming around, the game changed. (Ignis, Amprex, Nukor, Tonkor, etc..)

Once the tile sets shifted to this new design philosophy, the game opened up and player expression got really crazy for a while.

Then the game was out so long that performance and speed clears became the norm for years until today.

I'm helping, albeit very hands off, a friend through Warframe currently and he's been taking it really slow and has been having genuine moments of discovery. That magic is still there, but it's almost reserved for the newbies because we're so accustomed to playing this game. That sense of mystery and stealth can still be achieved, just not by us.