+1 for this feature.  I'm running lwIP with MQTT on iOS devices and it would be very helpful to cleanup / restart lwIP/TCP/MQTT.  I understand that keeping things simple and lightweight is a goal, but this sort of functionality would be stripped out by the linker when not used(hopefully) so I don't really see it bloating the footprint much, if at all.

I also would like to see this change.  Even if it was an optional configuration.  An example is we can run our application on real embedded hardware or on Windows/Linux using the tap interface.  When on the PC, the code needs to be able to exit the application and shut down.  Asan always complains about the tcpip thread memory not deallocated at shutdown, but that seem to be all that is left after closing sockets and network interfaces etc.  I guess all services would need a de-init function as well.

Currently, lwip does not offer cleanup functionality, so once it is initialized, it must remain initialized until the process terminates.  This helps keep lwip lightweight and simple, and works well in embedded environments that start lwip at boot, but it does not always work well for other use cases, such as libraries that offer functionality backed by lwip.

Those use cases would seem to be better served if lwip offered sufficient API to clean up all initialized state, and later re-initialize if needed.