score:1
Probably 1978 for the exact wording though phrases like 'law of health' have been used before.
No where can I find extensive or official reference to "the Lord's law of health" beyond
As to why the Word of Wisdom is referred to in this way it has probably nothing to do with the promises you mention but rather the commands to:
Abstain from:
Good/ordained:
I'll also echo OP comment that this was given
In consequence of evils and designs which do and will exist in the hearts of conspiring men in the last days, I have warned you, and forewarn you, by giving unto you this word of wisdom by revelation