I've paid for it once, I can see the receipt in Google play

This was early days when WhatsApp was still searching for a business model.

That business model turned out to be acquire critical mass and then be acquired, but I recall people talking about how WhatsApp was hemorrhaging money in the process.

Early adopters got life long free access... until eventually even the 99 cents was dropped.

Dropped because WhatsApp now can mine your metadata and make off like a robber at a convenience store.