If I recall correctly the maximum Noise Reduction Rating (NRR) for earplugs and earmuffs is around 30db. You can combine the two for a slight increase in hearing protection but you still hit a limit because of bone vibration.
Is there PPE out there to go even further beyond this? Where would it be commonly used?
This is partially correct, partially wrong.
As many have commented, flipping a signal by 180° cancels it out. However, this is only true for static noise though. Transient noise cannot be canceled out completely, because you would need to see into the future to know which signals to play to cancel out the noise.
The ANC headphones I own mainly cancel noise through passive shielding of the ears. The “active” noise canceling feature is not contributing a lot.