LwIP's TCP congestion control implementation was written back with RFC 2001 (1997), which has had two replacements: RFC 2581 (1999) and RFC 5681 (2009)

One thing that I know is out-of-date is setting ssthresh after a loss detection.  We have the original logic from RFC 2001 which is:

When congestion occurs (indicated by a timeout or the reception
of duplicate ACKs), one-half of the current window size (the
minimum of cwnd and the receiver's advertised window, but at
least two segments) is saved in ssthresh.

RFC 2581 updated this to (and I believe is still current in RFC 5681):

When a TCP sender detects segment loss using the retransmission
timer, the value of ssthresh MUST be set to no more than the value
given in equation 3:

      ssthresh = max (FlightSize / 2, 2*SMSS)            (3)

As discussed above, FlightSize is the amount of outstanding data in
the network.

Fast Retransmit

 1.  When the third duplicate ACK is received, set ssthresh to no more
       than the value given in equation 3.