What is the meaning of mainstay of treatment? does it include supportive treatments?

Matter of terminology. For example, antibiotics are the mainstay of treatment for pneumonia. Breathing exercises are supportive treatment. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!

Mainstay of treatment means the main or correct way of treatment. Give example: the mainstay of treatment for a bacterial infection is using antibiotic(s) then you can use supportive treatments such as Tylenol or Motrin for fever or body ache or intravenous fluid if you are dehydrated due to fever or poor intake due to the sickness. You can interpret mainstay as "must" and supportive as "may".