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I feel like Marilyn, Carolyn, and Evelyn are the three -lyn names that probably first made the -lyn trend a thing. (Cherilyn came a little later.) For -ley, it was probably Beverly, Shirley, and Lesley/Leslie. This probably started in the mid-20th century but didn't get really big until the end of the century.

But the OP is specifically referring to all the new variants, not the ones already commonly used.