What exactly is modern, seems very relative and ambiguous!!
I think the “modern WP stack” term started floating around when headless and serverless and static page generators and stuff like that started getting trendy so yah it was probably around 5 years ago or a bit longer.
Because “smart” web developers love to trash talk WordPress and how janky it is, not a 12 factor app, and all that so many times they use the term “modern” to mean “fixing some of the default crappy structure that is WordPress core”
Pretty subjective, really.