Tracking access time for structures only become available with Structure 8.2. This means the earliest tracking available occurs shortly after the first upgrade to 8.2 or higher. If a structure hasn't been accessed since that update, it is considered never accessed.
import java.time.Duration
import com.almworks.jira.structure.api.StructureComponents
import com.almworks.jira.structure.api.permissions.PermissionLevel
import com.onresolve.scriptrunner.runner.customisers.PluginModule
import com.onresolve.scriptrunner.runner.customisers.WithPlugin
@WithPlugin("com.almworks.jira.structure")
@PluginModule
StructureComponents sc
def sm = sc.getStructureManager()
def sps = sc.getStructurePropertyService()
def currentTime = System.currentTimeMillis()
def structures = sm.getAllStructures(PermissionLevel.VIEW)
structures.findAll { structure ->
def lastPolledMillis = sps.getLong(structure.id, "lastPolledMillis", 0)
return Duration.ofMillis(currentTime - lastPolledMillis).toDays() > 90
}