Thanks to the diligent folks over at "The Points Guy" blog, the answer to your question, going back ten years, is as follows:
- September 2022: 25%
- September 2021: 40%
- October 2019: 40%
- November 2018: 40%
- September 2017: 40%
- October 2016: 50%
- January 2015: 40%