As stated in LWIP documentation:
https://www.nongnu.org/lwip/2_1_x/multithreading.html

"When running in a multithreaded environment, raw API functions MUST only
be called from the core thread since raw API functions are not protected
from concurrent access (aside from pbuf- and memory management
functions). Application threads using the sequential- or socket API
communicate with this main thread through message passing."

https://www.nongnu.org/lwip/2_1_x/netifapi_8h.html
Is not present in current esp-idf / arduino-esp32 builds.
Using existing "private" TCPIP API, create a minimal struct to pass a message
to the LWIP thread instead of calling etharp_gratuitous directly.

See this comment and further discussion in the issue:
xoseperez/espurna#1877 (comment)

Use the same condition as esp-lwip.