Under the current Rules of Golf, the longest shaft a player can use in competition is 48 inches -- the precise length of Brooke Henderson's driver, which she uses to boost her length on the LPGA Tour. However, a potential USGA rule change might make the days of the 48-inch driver short-lived.
Golf Digest reports the USGA is considering reducing the maximum length of a shaft from 48 inches down to 46.5 inches. The USGA sent a notice to manufacturers in October letting them know of the potential change. The thought process is two-fold:
1. The USGA claims that just 1.1 percent of 600 professional players and 2.1 percent of 400 amateurs they surveyed use a driver length above 46.5 inches. Most pros typically use a driver in the 45-45.5 inch range.
2. Were more players to switch to a 48-inch driver, particularly pros, the USGA says they could gain as much as 6 yards each off the tee. (Hence why Brooke Henderson did it.)
Such a change would be consistent with the USGA's and R&A's 2002 Joint Statement of Principles, which said that “any further significant increases in hitting distances at the highest level are undesirable.”
A change isn't imminent, and it's unclear if any public reaction might sway a move to a new limit.