While, yes, clover seed was included in some grass seed mixes for many decades prior to the advent of selective herbicides in the years after World War II, this misses a critical point: such mixes contained all sorts of weed seeds as there was no effective means of controlling any of them even on grass seed farms. The fact of the matter is that before the 1940s there were no effective herbicides of any sort. Even in agriculture there were few options for getting rid of weeds. Tilling was, and in some places still is, a common method. 19th century farmers tried various salts and acids but few of these were effective and none of them were selective. The popularization of 2, 4-D - the first selective herbicide - after the War revolutionized not just lawncare by agriculture as well. This revolution did lead to clover's decline in home lawns but many people still like it and it is planted regularly. The difference is that in times past clover appeared whether it was wanted or not; now the homeowner has a choice.