Previous OTAs that are rootable

I'm getting a new g3 this week. I know the first thing the phone will do when I activate it is to do the OTAs. I want to root my phone. I want to know at which point do I stop the OTAs so I don't get stuck with unroot able version. I know the latest version, 6.0 is not root able. I do want to accidentally go to far.

Thanks

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