Don't quote me on this, but I don't believe that Phenibut or Gabapentinoids in general contribute in a meaninful way to respiratory depression. I've read some literatrue that indicates that there is a corellation between concurrent use of Opioids, Gabapentinoids (like Phenibut) and respiratory depression, but that it's not really a linear phenomenon; meaning that it's not going to get progressively worse the more Phenibut that you consume.
Take this with a grain of salt and as always, do your own research.