My 21 year old GE water heater has only been flushed once. I am afraid to do it now. Should I risk it or just ride it out to the end?

It was made in 1999. The bitch that sold me the house in 2005 told me that it was less than 2 years old. It has only been used regularly from 2005 to 2019 (about 14 years). The house has been empty since then with only about 3 weeks of use each year.