I am assuming that the new doctor did complete bloodwork? The current protocol is to discontinue & replace Metformin if kidney functions begin to decline, which is quite typical in people who have had diabetes for a long time, especially if they are older.
Other health conditions that lead to discontinuing it include impaired liver function and heart disease.
Sometimes, when you see the same doc for a long time, a certain complacency occurs and if things seem to be going well, adjustments don't get made. A new doctor, who doesn't know you so well, is more likely to rely on test numbers in establishing treatment.
My husband had this happen recently when his kidney function declined (was discovered by his primary.) It has taken several months to get things back in balance, but his A1C is now back below 7.0.

As to the Keto diet, I always remember my Dad, who was not formally educated, but was extremely well read and wise. His mantra in life was "all things in moderation." It is my first reaction whenever people talk about extreme diets or workout plans.
Did your doctor explain the reason for stopping metformin?