Technically, Tuesday’s announcement that the USGA and the R&A will
reduce the maximum shaft length from 48 inches to 46 inches counts as a
suggestion.
It’s what’s called a Model Local Rule, which is generally designed to give
tournament organizers or presumably any tour the right to implement a rule
limiting driver length.
But a spokespeople from the PGA Tour and the LPGA Tour confirmed its tours would implement the local rule in 2022, with the PGA Tour confirming Jan. 1, 2022 as the first date of enforcement.
The PGA Tour Player Advisory Council recently reviewed the subject and
we have concluded that the PGA Tour will implement the Local Rule on Jan.
1, 2022."
The LPGA Tour didn't provide a definitive timeframe with its response:
“[We] plan to implement the new rule at some point after the 2021 season.
We have already started communication with the small number of players
who may be affected by the change, to ensure they can make any necessary
adjustments in a timely manner.”
It was typical in the 1990s for drivers played on tour to be in the 43-inch
range, and now the majority of tour players use a driver between 44.5 and
45.5 inches.
In addition to Mickelson’s and DeChambeau’s interest in drivers at the current 48-inch limit, the LPGA’s Brooke Henderson also has played a 48-inch driver in the past.
Though some manufacturers have introduced drivers with shaft lengths longer
than 46 inches, few have been successful.
golfdigest.com：Golf’s ruling bodies take next step in limiting driver length
