I forgot that I had years of free uploads from my Pixel 2. That eventually expired. But I never had to pay for those uploads.
I also had 100 extra GB for two years for being one of the first Google Maps guides, populating the map with the first photos of businesses etc. When that expired, I did have to start paying. Google are smart - they got me addicted to having all my files right there wherever I was. It’s only $20ish per year for that tier, I have been happy to pay for it. I think this might be what happened to you. You may have had some sort of promotion that expired.