Played a mod of ET last year and there was leaning left and right. Out of curiosity, when was this introduced?
2003 with the game’s release.
If you are talking about the thirdperson animation of players leaning then this was introduced first in NoQuarter and later introduced in ET: Legacy as well.
The reason being that players leaning from behind a corner are seeing things that are behind that corner, yet players that are behind said corner don’t see the player, which is the literal definition of a wallhack. Therefore, thirdperson leaning animation has been introduced.
@KeMoN is correct. The leaning function in Wolf ET was taken from Rtcw, thus both SP and MP modes of the game allow a player to lean without the foes seeing them. However, at least in SP mode, the player can still take damage if bullets/projectiles are being fired while the player leans.