So I can't speak to the future of legislation. In my experience/opinion, there is a certain disconnect between reality and legislation, particularly when it comes to "dangerous things." So arguing rationally about moderator design and future legislation is a bit futile.
That said, I would contest the assertion that, "with all the slugs coming into the picture there’s not much of a difference between air rifles and powder burners." This is untrue, both in regards to airgun performance and airgun moderator design.
Forget all the "slugs" for a second, and lets go straight to the Umarex Hammer, the self proclaimed most powerful "production" PCP.
Grain Material FPS Energy (ft. lbs.) 180 Umarex® ARX® 1100 484
200 Lead 1055 495
250 Lead 1000 555
275 Lead 945 545
300 Lead 930 576
330 Lead (Lyman) 935 641
350 Lead (Lyman) 875 595
550 Lead 760 700
(ref.
https://www.pyramydair.com/s/m/Umarex_Hammer_50_PCP_Air_Rifle/4336) So excluding what is, in my opinion, a cheat using an exceptionally heavy projectile to gain more KE, realistically you'll want to be in the mid-high 800s and so the Hammer reasonably produces just shy of 600 foot pounds. That is about what the loads I carry in my handgun do, and I consider my and essentially all handguns woefully under-powered. It just isn't that much go juice, and the hammer promises only 3 shots at that power level. On the scale of firearms then, that isn't really even a blip. Take 5.56 for example, another cartridge commonly maligned as "inadequate" and "under-powered." Tell most hunters you're planning on taking a deer with 5.56 and they'll give you a really hard time for being "unethical" saying you need at least a .30 cal. 5.56 comes out the muzzle with about 1300 foot pounds, or twice what the Hammer has. (ref.
https://en.wikipedia.org/wiki/5.56%C3%9745mm_NATO) Then you start getting into "real man's" calibers, like 7mm Rem Mag:
Now we're into 3,000 foot pounds, and we're not into the realm of true big bore high power rifles, it can still be run on your basic "universal" centerfire rifle suppressor. (in this case a Silencerco Omega pictured)
And all this leads me to the yawning chasm in difference between firearm suppressor design and air rifle moderator design. (I've designed both) Substantial performance can be gained, in a firearm suppressor, by simply absorbing heat from the expanding gasses at the muzzle. Performance also can be gained by disrupting and diminishing the flame front so you eliminate the "first round pop" of a firearm suppressor, where oxygen in the design is consumed. And, of course, there are the operating temperatures.
Airguns meanwhile operate, not just at substantially lower temperatures, but lower pressures too. The max we run is, what, 4,500 PSI? Even the measley .22LR is rated for over 24,000PSI. (ref.
https://en.wikipedia.org/wiki/.22_Long_Rifle) So it is a substantially different system to try and slow that flow and absorb that sound, even out of a slug gun. All this means we can experiment with vastly different materials and architectures in airgun moderators. But we also have certain restrictions and limitations. Cross-flow, which is utilized as standard to gain performance in firearm suppressors, isn't really permissible here because our pellets are so easy to destabilize or steer. Meanwhile sound absorbing materials can be, and typically are, used as they won't be burned up. They are very very different things.
Just my 2c.
