I went for a bit of a nostalgia trip and installed the singleplayer portion of the original Call of Duty: Black Ops. And goodness me the campaign hasn't aged well. Which was honestly inevitable: cutting edge graphics are like the cutting edge of a knife. They only maintain that superb edge for a brief bit of time. Nowadays it's impossible to not notice all the ways it's dated: the rubbery action figure character models, the stiff facial animation, the lack of environmental detail compared to modern games, and the incredibly shallow gameplay that's about two steps removed from being a rail shooter. This was criticized even back on release, but one of the worst aspects of the singleplayer is its mind-numbing, non-stop intensity: there's absolutely no use of stillness or pacing, everything's at 11 all the time. Nonstop gunfire and explosions, constant shouted dialogue and bombastic music create an audiovisual vomit that's almost nauseating to play for more than 20 minutes at a time. So yeah, there's literally no reason whatsoever to play the singleplayer in this decade.
The much better aged, but still aged, part of the singleplayer is the Zombies mode, which maintains a much greater level of enjoyment as a simple arcade shooter. It's got a sort of crass, late 00's to early 10's meme charm to it. Like the devs just went "nazi zombies, teleporters, mad science, national stereotypes, lolderp". It's enjoyable enough, though the movement controls feel rather sluggish compared to how fast the enemies get. It gets pretty repetitive in singleplayer, once you unlock the special weapons you've basically seen everything, and then it's just going through the motions. I never got to play it in properly organized multiplayer, and it's probably a lot more fun there because it was designed around that.