Previous value must come from env var. This change has to be made in coder/coder to take advantage of this.

CODER_PARAMETER_PREVIOUS_<hash>

Unfortunately reading from tfstate requires the block to be a resource block. This is because a data block only has access to the Read context.

https://github.com/hashicorp/terraform-plugin-sdk/blob/main/helper/schema/resource.go#L287-L354
Read

// The *ResourceData parameter contains the state data for this managed
// resource instance or data resource instance.

Update

// The ResourceData parameter contains the plan and state data for this
// managed resource instance. The available data in the Get methods is the
// the proposed state, which is the merged data of the prior state,
// practitioner configuration, and any CustomizeDiff field logic. The
// available data for the GetChange* and HasChange* methods is the prior
// state and proposed state.

coder/coder PR in progress coder/coder#17696