What you're looking at is a comparison of the front brake pads from the FK2 Type R, FK8 Type R, facelift FK8 Type R and the FL5 Type R.
The FK2 Type R pad uses a semi-metallic brake compound, developed to give a good balance between performance and durability, offering great stopping power even at high-temperatures as well as high wear resistance. This compound isn't without issues; it generates a high amount of brake dust, produce quite a lot of noise (even more so when cold) and can cause large amounts of disc corrosion due to the high metal content.
Moving forwards, the 2017-2019 FK8 Type R pad uses a compound that visually looks similar, however changes were made to ensure better fade-resistance, inspiring confidence for track driving, and to improve performance at high temperatures. This pad also featured a chamfer on the leading and trailing edge of the pad for the purposes of reducing noise and vibration.
The facelift FK8 Type R brake pads were changed yet again for this variant. Honda removed the chamfered edge on the pad and once again changed the compound to have improved fade-resistance and increased performance at high temperatures. Visually this brake pad seems to have significantly less metal content than the previous 2 iterations.
A brake pad compound very similar to the 2020 FK8 facelift is used on the FL5 Type R however the chamfered edge found on the 2017 pre-facelift FK8 is reintroduced. The brake compound found on the FL5 pad is the most advanced of the 4, with further improvements to brake fade and performance at high temperatures as well as reductions in the amount of brake dust produced.
Although Brembo do not release the exact contents of the compound it is easy to see visually that the largest change has been the move away from compounds with a higher metallic content and towards compounds with more organic materials such as rubber, glass fibre and kevlar.