It was netherite gear and I died in the middle of a tunnel down into a mine that I had made and double checked around, breaking blocks in the area and looking everywhere, so it certainly wasn't destroyed by anything like cactus. I can say with certainty that the armor and shield disappeared due to some sort of glitch, because every other item in my inventory was there on the floor.
I did die in a similar fashion later, though my armor dropped that time, so the glitch seems to be inconsistent.
Had my equipped armor and shield disappear when I died by hitting a wall while flying with elytra in a completely isolated tunnel on a realm. The rest of my inventory was there in a suspciously neat pile on the floor, not spread out like usual when one dies. The armor and shield that I had equipped however just disappeared for some reason. I checked around everywhere and there was no way the gear could have gotten destroyed or stolen, since there was nothing around the tunnel other than solid blocks and nobody else was online.
On the 1.19.11 Hotfix