You might get more response to this over on the [dev] list. Who knows, maybe Wally will move it there?
As Stefan intimated, the most obvious change with 4.6 is the move to UB in the Mac world, which entails (for most people) a major change in the development environment. DDZ wrote an extensive article on this, I have a link to it on my home machine but I won’t be back home for another week. It’s somewhere here on the C74 site.
In terms of the API per se, I didn’t notice a lot of change. There’s now a collection of byte-swap macros&functions, but they may have been introduced back with 4.3. In the past I tended to use CW utility libs for this sort of thing, but now that there is no longer an IDE that can single-handedly build both Mac & Windows externals, ext_*.h is probably the better approach.
Almost forgot (ack!!): the pattr API is fully documented & integrated. That’s pretty major.