[..]
If I remember correctly, Sylvain started to make things work that he needed
when porting the updated PPP sources. It doesn't necessarily mean there was
an issue, but I guess he didn't want to implement proxy ARP support in the
lwIP ARP layer unless he needed it...
Indeed. If it would be implemented proxy ARP control plane should live
in the netif core code and data plane in the ARP layer. There is no ARP
proxy support per se on the PPP protocol, PPP layer do no more than
setting a few internal flags and asking the system to set/remove an ARP
proxy entry.
Anyway, ARP proxies seem to be a mostly dead thing nowadays, and I
consider that a good thing because it's a huge layer violation. However,
if some day ARP proxy support materialize in the ARP layer I will of
course make it works for PPP since it is just a couple of flags and
functions calls.