No matter what wording is used, you are not going to stop that one driver from wanting to take advantage of any situation.
Maybe he slows to 52% instead of 50%. Or closes within 5/8 of a car length instead of 3/4. We all know when we do it wrong. Simply put, in a yellow condition, local or full course, do not improve your position, do not pass, do not do anything stupid, stay in complete control of your car. Do not endanger safety people.
Just as we all want respect for each other as drivers, we need to give more respect to the people who make our fun possible.
Under a full course yellow if a driver doesn't "take advantage", close the gap, I'm sure someone who is behind someplace along the way is going to have a little talk with said driver. Point being no one owns the yellow flag gap. Same for standing or waving yellow flag, no one owns the gap.
Point B. Don't remember if it was Council or SCCA at the Farm several year ago, full course yellow for at least 3 laps when coming off T1 a SRF lost control, went inside track and crushed legs of 1 or 2 workers attending to peer driver. Existing rule or proposed do nothing to change/improve said situation.
Point C. Agree 100%, but it's proven at every race during yellow flag situations, respect is not conveyed by drivers when on track.
How fast do I need to reduce speed?
What's to stop me from just taking a foot off the gas and coasting as much as I can?
Point A. You have a functioning Speedometer on your dash. You read your speed is 100 mph, you reduce your speed by 50% to 50 mph. Take your foot off the gas pedal.
Point B. Nothing and the corner worker recognize your lack of speed reduction, call in your car number and you get a black flag drive through. The black flag will send a message and folks will learn in a hurry.