Minecraft wiki entry for Pressure_Plate, associated text: "Polished Blackstone Pressure Plate" is the current longest name for a block tied with Cracked Polished Blackstone Bricks (34 characters with spaces, 31 without)
It briefly lost its place between Java Edition 1.17 snapshots 20w45a and 21w05a, where it was surpassed by variants of the Copper Block:
Waxed Lightly Weathered Cut Copper Stairs (41 characters with spaces, 36 without in first place)
Waxed Lightly Weathered Cut Copper Slab (39 characters with spaces, 34 without in second place)
Waxed Semi-Weathered Cut Copper Stairs (38 characters with spaces, 34 without in third place)
Waxed Semi-Weathered Cut Copper Slab (36 characters with spaces, 32 without in fourth place)