It looks like that you lose the function:espNowFloodingMesh_isSyncedWithMaster()

What do you mean by lose it?

Like @tang30000 I am trying to use an example. He is referring to method espNowFloodingMesh_requestInstantTimeSyncFromMaster() which is not declared in header file. IDE will snow an error and compile will also fail. Examples use this method and it's not clear if this can be dropped.

See this line:

espNowFloodingMeshLibrary/examples/start-network/slave/slave.ino

Line 22 in d62739d

espNowFloodingMesh_requestInstantTimeSyncFromMaster();

src/main.cpp: In function 'void setup()':
src/main.cpp:49:3: error: 'espNowFloodingMesh_requestInstantTimeSyncFromMaster' was not declared in this scope
   49 |   espNowFloodingMesh_requestInstantTimeSyncFromMaster();
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/main.cpp:52:10: error: 'espNowFloodingMesh_isSyncedWithMaster' was not declared in this scope; did you mean 'espNowFloodingMesh_syncWithMasterAndWait'?
   52 |   while (espNowFloodingMesh_isSyncedWithMaster() == false)
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |          espNowFloodingMesh_syncWithMasterAndWait
*** [.pio/build/nodemcuv2/src/main.cpp.o] Error 1

same, as well espNowFloodingMesh_begin(ESP_NOW_CHANNEL), is not working. It's defined like that:
#ifndef USE_RAW_801_11
void espNowFloodingMesh_begin(int channel, int bsid) {
#else
void espNowFloodingMesh_begin(int channel, char bsId[6]) {
#endif