One thing to think about is that something actually becomes an exploration game, if it has the basic "exploration" as JDR13 described it (and while not present in PacMan, it is present in Wolfenstein3D or even it's predecessor Castle Wolfenstein:
https://www.youtube.com/watch?v=tJNAXh-LVnk) and a lack of other features.
Think about Wolfenstein3D for example. I don't think lots of people would claim that it's a game about exploration, because killing Nazis is the focus of the game.
But...let's imagine that you remove all the nazis. Or only sprinkle them in a couple of rooms, so that the combat doesnt matter anymore. Now it would become an exploration game. Find your way out of the maze and the keys to unlock the doors. Score points by finding secret passages.
Maybe similar to the so called "Walking Simulators" (even though it's kind of used as a joke description). They are Walking Simulators because you can't do much in them. They are mostly about moving around, with some story (and exploration). If you'd add first person combat, they are suddenly First Person Shooters and the walking...well...you walk in all shooters, right? But a shooter isn't a Walking Simulator as it has other features as well.
Some Walking, Story and Exploration can be found in pretty much every game (exceptions exist).
The one way a game becomes about one of these aspects is when it does focus on it a lot. Skyrim or Dead State focus a lot on exploration for example (well...most Open World Games do).
The other way the game becomes about one of these aspects is when it lacks any significance in other elements. Telltale Games might be a good example which I would hardly call adventure games, because they don't have any puzzle elements which adventures usually have. Tahira is another good example. While they claim it's a RPG, it lacks any RPG elements in my opinion. I'd call it a tactics game (and a good one).
Ofc the thing is, that some Genres are dominated by one mechanic. If you play a poker game, the game will be about a poker mechanic.
If you play sokoban or tetris...well, that's a puzzle game.
But genres like RPGs have many parts of other genres (and now also so many genres have part of traditional RPG mechanics) that it also carries over assumptions.
However just because RPGs traditionally have exploration aspects, and shooters only developed that during the 90s, that doesn't mean that RPGs have to have better "exploration" than shooters or good exploration at all for that matter.