Is this instruction from the LORD the first time a distinction is made between clean and unclean animals?
The concept of clean and unclean animals was not new from the biblical perspective. As far back as the account of the flood (Genesis 7:2), there is a distinction made between the clean or unclean animals that were to go into the ark.
2 Parlez aux enfants d'Israël, et dites: Voici les animaux dont vous mangerez parmi toutes les bêtes qui sont sur la terre.