Another clear case of the FIA overreaching.
#1. Using exhaust pressure to generate downforce is neither a new concept nor a creation of the hyperactive minds in F1. It has been being used by a great many forms of motorsport for a very long time.

#2. The FIA have no idea how to confirm compliance. The system works by increasing fuel flow and retarding ignition when off-throttle so the fuel-air charge is ignited too close before opening of the exhaust valve for all the charge to be consumed before being squirted through the exhaust port.

This differs from a "normal" off-throttle condition only in a manner of degree. To enforce this objectively and empirically, the FIA will have to specify the precise throttle position where "off-throttle" mapping in invoked
and either the amount of fuel that can be flowed or the minimum timing advance when in an "off-throttle" condition
or some combination of the three.
I wouldn't want to be the FIA bloke charged with determining those exact numbers, which then must be applied to four different makes of engine, with some very different design philosophies and individualised for the 12 teams. This brings to mind the title of the Molly Hatchet song,
Flirtin' with Disaster.
Speaking of
Flirtin' with Disaster, that's exactly what imposing any rule on F1 in the interest of "greenness" is. If you know anything about the history of the movement, they are never -- will never be -- satisfied. If 35% reduction is good, 50% must be better, yes? And if 50% is better, it's only a matter of time until they will drive F1 to this:

So if you would like to see Formula 1 racing driven from existence, feel free to be a green
sheeple. Once there are enough of you, even the soap box derby cars will be banned for fear the friction they create with the air will lead to further global warming.