Are there any kind of statistics how common are low MTU links (MTU is below 1492) in the common Internet?
Perhaps this paper and its references are what you need, it includes graphs of found sizes and links to datasets. They report that 98.4% of reachable servers in their survey have a PMTU >= 1500, and have other details for various sizes and metrics.
Custura, A., Fairhurst, G., & Learmonth, I. (Accepted/In press). Exploring usable Path MTU in the Internet. In Network Traffic Measurement and Analysis Conference, TMA 2018 IFIP Open Digital Library.