Depends on what you mean by "downgrades." For a while, my wife's 2019 M3 wasn't on the FSD Beta stack while my 2018 M3 was. I would regularly drive to work, 20+ miles mostly on highway, in both cars, so I can say for certain there are differences in how FSDBeta drives on the highway vs the non-FSDb stack. I personally find the FSDb stack better at highway driving than the non-FSDb, but your mileage may vary.I have a 2022 with basic Autopilot. On the out of the factory software version I had one mild phantom braking incident, but after an update, and through many more, I have had zero. This is on SoCal freeways in clear weather.
Maybe going into FSD beta downgrades the highway stack as well?
My understanding these differences are mostly due to the FSDb stack being Vision-Only, while the other stack as of recently was not, but it seems like they're finally moving the old stack onto Vision-Only too, so likely the differences I noticed will go away. (I personally can no longer test this, as my wife's M3 was finally moved onto FSDb.)