NCS v3.3.0: OPENTHREAD_TREL=n build broken when OPENTHREAD_ZEPHYR_BORDER_ROUTER=y

Question

Summary 
 With CONFIG_OPENTHREAD_ZEPHYR_BORDER_ROUTER=y and CONFIG_OPENTHREAD_TREL=n , the Zephyr/NCS build fails to link: 
 ld.bfd: ... openthread_platform.a(trel.c.obj):
 in function `socket_received_cb':
 trel.c:(.text.socket_received_cb+0x40):
 undefined reference to `otPlatTrelHandleReceived'
 
 The reason is that zephyr/modules/openthread/platform/trel.c is gated on CONFIG_OPENTHREAD_ZEPHYR_BORDER_ROUTER , not on CONFIG_OPENTHREAD_TREL . So the TREL platform glue compiles even when TREL itself is disabled, and references otPlatTrelHandleReceived which is only emitted by the OT core when OPENTHREAD_TREL=y . 
 This blocks any attempt to use the BR feature group without TREL, which is exactly the configuration we need (see companion DevZone post devzone-trel-mle-wedge.md — TREL=y deadlocks the OT thread on NCS v3.3.0 + OT pin a03011cf7 ). 
 
 Environment 
 
 nRF Connect SDK: v3.3.0 
 Zephyr: bundled v4.3.99 from the NCS workspace 
 OpenThread: OPENTHREAD_SOURCES=y , upstream pin a03011cf7 (DevZone #127950 prevents using the prebuilt Nordic library here) 
 Board: nrf54lm20dk/nrf54lm20a/cpuapp (also reproduces on nrf5340dk/nrf5340/cpuapp ) 
 
 Failing Kconfig (minimal):

CONFIG_OPENTHREAD=y
CONFIG_OPENTHREAD_SOURCES=y
CONFIG_OPENTHREAD_ZEPHYR_BORDER_ROUTER=y
CONFIG_OPENTHREAD_TREL=n
 
 Root cause 
 In zephyr/modules/openthread/CMakeLists.txt (or the equivalent NCS overlay) the openthread_platform library unconditionally adds platform/trel.c whenever OPENTHREAD_ZEPHYR_BORDER_ROUTER is set:

zephyr_library_sources_ifdef(CONFIG_OPENTHREAD_ZEPHYR_BORDER_ROUTER
 ...
 platform/trel.c
 ...
)

platform/trel.c calls otPlatTrelHandleReceived() from its receive callback, but that symbol is only emitted by OT core when OPENTHREAD_TREL=y . 
 
 Suggested fix 
 Tighten the gate:

-zephyr_library_sources_ifdef(CONFIG_OPENTHREAD_ZEPHYR_BORDER_ROUTER
- platform/trel.c
-)
+zephyr_library_sources_ifdef(CONFIG_OPENTHREAD_TREL
+ platform/trel.c
+)

trel.c belongs to the TREL platform implementation; gating it on OPENTHREAD_TREL is semantically correct and resolves the link error. 
 A second small change is helpful but optional: in zephyr/subsys/net/l2/openthread/openthread_border_router.c , openthread_start_border_router_services() calls trel_plat_init() unconditionally. With the CMake fix above, trel_plat_init is no longer compiled, so the wrapper now also fails to link. Either: 
 
 gate the call on IS_ENABLED(CONFIG_OPENTHREAD_TREL) , or 
 provide a weak no-op trel_plat_init returning OT_ERROR_NONE in the platform glue when TREL is disabled.

Workaround we are running 
 In the application's CMakeLists.txt :

if(CONFIG_OPENTHREAD_ZEPHYR_BORDER_ROUTER)
 target_sources(openthread_platform PRIVATE
 ${ZEPHYR_BASE}/modules/openthread/platform/infra_if.c
 ${ZEPHYR_BASE}/modules/openthread/platform/udp.c
 ${ZEPHYR_BASE}/modules/openthread/platform/mdns_socket.c
 ${ZEPHYR_BASE}/modules/openthread/platform/border_agent.c
 ${ZEPHYR_BASE}/modules/openthread/platform/dhcp6_pd.c
 )
 if(CONFIG_OPENTHREAD_TREL)
 target_sources(openthread_platform PRIVATE
 ${ZEPHYR_BASE}/modules/openthread/platform/trel.c
 )
 endif()
endif()

Plus an app-side stub for the missing-when-TREL=n symbols:

# if !defined(CONFIG_OPENTHREAD_TREL)
 void otPlatTrelHandleReceived(otInstance *aInstance, uint8_t *aBuffer,
 uint16_t aLength,
 const otSockAddr *aSenderAddr) { ... }

otError trel_plat_init(otInstance *instance, struct net_if *ail_iface) {
 return OT_ERROR_NONE;
}
#endif

This is a cosmetic, app-local workaround for what should be a one-line CMake fix upstream. 
 
 Reproducer 
 A minimal app with the Kconfig snippet above and any OPENTHREAD_ZEPHYR_BORDER_ROUTER=y configuration will fail to link in stock NCS v3.3.0.