If I want to use the "transcoding protocol" to decompress data directly into a Matrix{Float64} I would use unsafe_wrap to convert the matrix to a Vector{UInt8} in this new version of the API? I don't fully understand what the warning in the docstring of unsafe_wrap means and if this is safe to do:

the programmer is responsible also for ensuring that the
underlying data is not accessed through two arrays of different element
type, similar to the strict aliasing rule in C.

It is generally not a good idea to use unsafe_wrap on existing julia memory. It is really primarily for C memory only.

However, the IO protocol in Base is mostly based around calling unsafe_load/unsafe_store!/unsafe_copyto! because of the reasons you stated, about wanting to do IO directly to and from arbitrary types. The Memory representation might even be an okay way to represent doing that, as a means of creating a fat pointer for adding memory-safety to IO. But it didn't seem like this package was testing any of that.

But anyways, that documentation is sort of a lie there, since that is not really how anything seems to be implemented here. Nor would it be even be that sensible to implement that way. Everything is required to go through the Buffer object (a Vector) and only that object ever gets converted to Memory (basically, a slower, less-safe view of said Buffer). For example: