That sounds like a big disaster waiting to happen. You've been on prednisone a long time, so your adrenal glands are probably offline. When I started tapering down from 5 mg, my rheumatologist told me to taper 1 mg per month. I also take weekly Actemra injections, so my PMR and GCA are controlled by the Actemra. My biggest risk with each 1 mg step down is running into adrenal insufficiency. If you go from 5 mg to 0 in one step, you could feel really bad in the best case, and maybe go into adrenal crisis in the worst case. You could also have a bad PMR flare. Most people who are not taking a biologic drug like Actemra or Kevzara taper a half milligram a month in order to avoid a flare or adrenal insufficiency.

Waaaaay outside any norm.
Be careful, and aware of adrenal insufficiency and adrenal crisis. Unless I am missing something, I agree with the “…disaster waiting to happen” reply by someone else.
If possible, you should get a second opinion stat.