yes State of Decay was supposed to have co-op, this article doesn't tell the whole story but here (
https://www.destructoid.com/state-of-decays-multiplayer-co-op-has-been-nixed/).
Before State of Decay came out they did blog posts on the undead labs site and discussed how it will be a zombie community management game with co-op. However, a few months before release they said they weren't able to finish co-op in time and will release it on a later date. I can't find the reddit post that unraveled this being a lie, but Undead Labs did go through a development hell with State of Decay. You notice that there was progress made with co-op on the game too if you play it right now, with the communication and emote mechanic in the game that solo play only use is telling people to stop or follow. Yes they did have the class 3 and 4 projects, but I don't recall them really elaborating on those things till game released and stating why some things were missing. (full disclosure I followed the original game from it's initial announcement and through the dlcs they released for it)
Overall, co-op was one of a few things they promised and did work on the original game but either got removed or half baked during development hell. I hope State of Decay 3 improves on the co-op though, cause the having to stay near the host in 2 is annoying.