Early decline of the shooter.
Half-Life
Far Cry
Counterstrike
Doom 3
Battlefield
Crysis
Bioshock
What is your definition of decline?
There is a big difference on this reasoning: "I will make my game linear because I can control this encounter this way and it will be fun, to have this puzzle there, to have this enemy appearing there, to have marines ambushing the player and giving him a hard time.", to "I need to include shooting galleries, cover shooting, aim assist, recovering health, static enemies that are easy to headshot and are mostly harmless, make shooting gallery levels because otherwise I won't sell."
Depends of what your definition is, with exception of Biocock (Pure decline garbage in comparison with System Shock 1 and 2), those are really diferent games with diferent proposals, I don't much of a common ground between them to claim there was a decline and conscious popamole design on them.
If your definition of decline is broad like linear games are decline, games with scripted moments are decline, why not go wild as you can and do like some people here on the codex that see the whole shooter genre as awful decline? If your personal taste is non linear, labyrinth level style games that is one thing but a definition that placed that standard on all FPS shooters is going too much on subjective territory.
I had great fun playing Fear, Half Life, Doom, Thief and Deus Ex, they are completely different games that have completely different proposals and completely different level design, a Half Life level design on a Deus Ex game would be awful but it is acceptable on a dumb shooter. I don't consider Fear decline because it isn't like Doom, they both have crazy good gameplay for different reasons. Only progressive gamming journos dipshits would consider a game good or not based on the release date.
To me, the requirements for a good run and gun shooter are speed, skillful aiming and movement, challenges that demand skill to solve and keep you surprised, weapons that feel good to shoot, all the rest depends on the proposal of the game. I wouldn't play a linear Thief or Deus Ex because their gameplay isn't suited to that and a Fear or Half Life style of game would be far different if they had non linear levels.