The WAV format never had a licence.

By default that means that, depending on your jurisdiction, it is very likely proprietary. No license mean it does not impart any rights to you. Some places do have a "by default this is owned by the public domain", but I don't think that's a standard.

To be a “proprietary”, a file format needs to either be secret (in part or in whole) or require a licence. WAV satisfies neither of those criteria. It is not proprietary.

The WAV format itself is owned and was developed by IBM/MS. I don't know where you live, but that is wildly incorrect on how copyright law works in most places. By default, the creator owns the product and all rights to its use. Then they may choose to allow others to use it within a license or other agreement. In this case, IBM/MS allows anyone to use the file format, but it is within their rights to also disallow this at any time. Here you can see for yourself, it is a proprietary format. Something not being proprietary is an exception, you have to explicitly make something that way. For example Ogg Vorbis is open source, open licensed and patent free.

It would be fair to say that it was proprietary up until it was reverse engineered...

Being reverse engineered absolutely does not invalidate the proprietary ownership of a product. Compare this to the Ocarina of Time decompilation project; just because the game has been fully reverse engineered, does that mean that Nintendo does not still own the game and all rights to it?