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I haven't played 40k since 8th edition and I was wondering if it was the case that you could return some units back onto the board when they were destroyed, kind like in some of the formations in 7th and before.

Some factions have stratagems that can be used to respawn a dead unit. Like the tyranids Unending Swarm detachment contains the Unending Waves stratagem which lets them bring back a squad of small nids like gaunts or gargoyles.
But there's a balance update that means you can only use these stratagems once per game.

I think it’s a major part of Genestealer Cults as well.

It's also a feature of the Astra Militarum Combat Patrol.

Kroot get it as well with Join The Hunt.

Unending Swarm, Combat Patrol, and Join the Hunt are all detachments?

Unending Swarm is a detachment, Combat Patrol is a game mode and Join the Hunt should be a special rule.

Cool, thanks