It is indeed an old problem (small fixed size buffers) but it hasn't really been fixed, they only increased the buffer size. You'll only run into this on high bandwidth, high latency connections, e.g. between NYC and AMS - you'll be limited to about 300mbps with stock OpenSSH.

https://www.psc.edu/hpn-ssh-home/hpn-ssh-faq/ has a patchset that fixes the problem.