Since version 1:5.9p1-1 the IPQoS option is ignored for AF_INET sockets.
Running ssh -F /dev/null -vvv -o IPQoS=lowdelay somehost true gives
this.

debug2: client_session2_setup: id 0
debug2: fd 3 setting TCP_NODELAY
debug1: Sending command: true

Downgrading just openssh-client to 1:5.8p1-7 gives this.

debug2: client_session2_setup: id 0
debug2: fd 3 setting TCP_NODELAY
debug3: packet_set_tos: set IP_TOS 0x10
debug1: Sending command: true

Unfortunately I cannot tell you precisely what broke. Looking at the
binary I can see that the relevant log message is contained, so the
relevant code is still present in the binary.

Curiously, exactly the opposite behaviour exists for AF_INET6:

With 1:5.8p1-7:

  % ssh -vvvo 'IPQoS ef' -6 -Snone vera true 2>&1 >/dev/null | egrep tos || echo no match
  no match

With 1:5.9p1-2:

  % ssh -vvvo 'IPQoS ef' -6 -Snone vera true 2>&1 >/dev/null | egrep tos || echo no match
  debug3: packet_set_tos: set IPV6_TCLASS 0x08