I’ve noticed this too. My travels lately have been divided between DL and WN. Never hear the lav announcement on DL and ALWAYS hear it on WN.
My take on the no lines at the lav thing is that it’s pilot driven at least on WN. I notice that the pilots many times repeat the warning just after the FA’s have already announced it.
I’m thinking the pilots want to be able to jump into the lav....although they’re not supposed to be leaving the cockpit.....without having to deal with a long line.
They used to turn on the seat belt sign to free up the lav. Nowadays it’s the “no lines at the lav” law.