Goretex filters probably (used to be) or a similar Aquaphobic filter. Some newer MEMS mics are less susceptible to moisture too.
I know they were quite advanced in respect of acoustically transparent micrometer thickness film layers. They did it for the receiver with the ‘Wax Sentry’ a while ago.
Receivers are still the weak point in my book. I’m not entirely sure why the diaphragm ’needs’ to be fully enclosed. From an engineering point of view the capsule could be made with one fully dynamic (wipeable) face that just ‘stuck out the back’ of the ITE/CIC/RIC mould. Thuras function could come from internal venting within the mould. Hiding the driver surface away just makes the issues worse in my book: unless it’s sitting directly into wet wax (but that could just wipe off) or into a canal wall. A wipeable dynamic gold plate (or Mylar encased) occupying a 4mm oval on the rear face would seem to be a good solution.