Patch 7.3.5 introduces a new scaling system similar to what we have in the Legion zones, so every mob will scale to your level now in older zones too. Fortunately, the scaling is capped at certain levels for each expansion, you won't be fighting level 110 mobs at Barrens.
So if you farm old materials with a level 110 character, this change shouldn't effect you since you will one shot everything anyway. However, this will make it harder to farm with lower level characters.
Skinning and Tailoring
Based on the tests I did on PTR, there was no change in cloth or leather loot tables. So, even if the mobs scale up to a higher level, they will still drop the same leather and cloth as before. The level 58-60 mobs in Barrens still dropped Light Leather and Linen Cloth.
Many of the old enchanting materials that were used to level enchanting between 1 and 300 were replaced with new materials as part of an effort to simplify things. This will make Enchanting a bit easier to level since 100-300 will basically use the same materials instead of using 5 different kind of dusts and essences.
Currently these are the materials used for 1-300: