I was curious but the most recent data I could find was from 2017 when there was a mix of CDNs at initcwnd=10 and initcwnd>10:
https://www.cdnplanet.com/blog/initcwnd-settings-major-cdn-p...
Currently Linux still follows RFC6928 and defaults to initcwnd=10:
https://github.com/torvalds/linux/blob/v6.11/include/net/tcp...