I can't offer any data myself, but I can suggest that chasing up the reason that more "modern" runtimes like Go's and Rust's don't bother to expose SO_KEEPALIVE support — i.e. the discussions that ensued when someone proposed adding this support, as they certainly did at some point — would be a good place to find that data cited.
I can point out the obvious "analytical evidence", though: note how all the platform APIs that did expose SO_KEEPALIVE are from the 90s at the latest — i.e. before the proliferation of L4 middleboxes. And note how modern protocols like Websockets, gRPC, and even HTTP/2 (https://webconcepts.info/concepts/http2-frame-type/0x6) always do their own L7 keepalives, rather than relying on TCP keepalives — even when there's no technical obstacle to relying on TCP keepalives.
I can point out the obvious "analytical evidence", though: note how all the platform APIs that did expose SO_KEEPALIVE are from the 90s at the latest — i.e. before the proliferation of L4 middleboxes. And note how modern protocols like Websockets, gRPC, and even HTTP/2 (https://webconcepts.info/concepts/http2-frame-type/0x6) always do their own L7 keepalives, rather than relying on TCP keepalives — even when there's no technical obstacle to relying on TCP keepalives.