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+```release-note:feature
+Added `tfsdk.PreserveState()` as a built-in plan modifier, which will automatically replace an unknown value in the plan with the value from the state. This mimics the behavior of computed and optional+computed values in Terraform Plugin SDK versions 1 and 2. Provider developers will likely want to use it for "write-once" attributes that never change once they're set in state.
+```