Since you can fall overboard and drown in 0 knots of wind.
Why have a flag that gets used when ... "conditions warrant".
It's always warranted!

The discretion to fly or not fly the "life jacket required flag" shifts the responsibility from the skipper to the PRO.

The OA is better off being able to state. "We never fly this flag under any circumstances or we don't own this flag when the spouse of the drowned sailor wants to know why NO ONE REQUIRED him to wear a life jacket.

Junior Regattas will put the requirement in the SI's and so it's in effect when you leave the beach in 0 to 30. The difference is that no discretion is allowed.

Just like PU declaring that sailors have an expectation of safety from the PRO and the OA... or
Defining some boats on the course as Safety boats....allows some novice sailor to decide... Well... it's OK to race because they have safety boats that will bale my butt out. or
The life jacket flag up or down shifts some responsibility from the skipper to someone else ....

Nothing good can come of it!