There are other sources but the reason I linked corelle directly is because they admit that pre-2000s plates have trace lead amounts. They use weasel wording sure, but it says it.

Are Corelle® products lead-free?

… Corelle was first introduced by Corning over 50 years ago and in 2000 started to be manufactured by the company known today as Instant Brands.

… Before 2000, and before tighter lead content safety regulations, a small amount of lead was an ingredient in the decorating process of many household products.

There are other sources, but most of them are websites ive never heard of. Figured corelle was the easiest source to link.