I decided to compile the latest OpenSSH, largely because I can, and partially because I was trying the throughput patch. It didn’t work out too well, which might be due to my aging hardware. I discovered that after removing Debian’s package and compiling and installing OpenSSH from source, I no longer had a running sftp-server. After some investigation, I found that although it was attempting to load the sftp-server subsystem in /etc/ssh/sshd_config, the executable was missing. I copied it from the compilation directory to the directory set in sshd_config, set the permissions, and restarted sshd. It works now!