"using `in ref` is deprecated" in client code while the module's unittest does not warn

[kdevel]

After switching to dmd v2.105.3 I get this warnings almost 
everywhere. Do I really have to fork msgpack-d in order to get 
rid of these warnings?

Now I replaced every "in ref" with "in" in my own code but I do 
not add that preview flag to the compiler invocation. Is that 
safe?