Squad has a zombie mod. Not really sure how often it's played. Unfortunately don't have the game installed right now, so can't see if there's any servers running it.
https://steamcommunity.com/sharedfiles/filedetails/?id=2842239012&searchtext=zombie
As someone else pointed out STALKER. And there's also STALKER Anomaly/GAMMA (a massive modpack for Anomaly) for basically an infinite replayable sandbox. But probably would benefit from playing at least 1 normal STALKER game first.
Ever played any of the stalker games? Kind of old but may scratch that itch.
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