I'd expect most serious web servers support it. I've written one that does (workerd), it's not too hard. That said, it's tricky to use if the server also does

I'd expect most serious web servers support it. I've written one that does (workerd), it's not too hard. That said, it's tricky to use if the server also does TLS termination... then you need kTLS, which is a much bigger can of worms.