i think there are many aspects that make waw and bo1 zombies retro. the most obvious one being its dated graphics. but its more than that. the games simple pick up and play nature is what really dates this games age. nowadays fps games and cod zombies are as complicated as is. you have to pick your loadout, check which abilities you are gonna use, endless amounts of leveling grind to achieve skins or upgrades. thankfully bo7 zombies has classic mode but the game doesn't feel like its designed around it. even modern boomer shooters aren't saved from this nonsense. in the new doom games its not even about finding weapons as you go through maze like levels. there's upgrade systems for each and every weapon in your arsenal.
just take me back to the simple days of fps games where i started out with a pistol in my hand and my objective was to shoot things. and thats what classic cod zombies does so well. all killer, no filler. all you need to know in classic zombies is that theres zombies coming your way and you have to fight them off and rack up points to be better weaponry and perks which are so dead simple to understand their perk jingle tells you all you need to know about their usage, instead of having to read through text to understand what specific upgrade does what.
I feel retro gaming as evolved more than being referred to anything from the 8 bit, 16 bit, or early 3D era of gaming. A lot has changed over the past 16 to nearly 20 years since these games released. The 7th gen era to me is getting pretty retro to me. Also factor in that th original 360 didn't even launch with an HDMI cable but instead an A/V one. While the 7th gen era did start a lot of modern gaming trends. Games still were made the old fashionway. no insane budgets and able to come out as quickly as they came. You could have an entire trilogy be developed for one single console generation. Now games take 5 or more years to come out. that is like more years than you stay in HS.