In Mac OS X you can (or could at one time) attach extra meaningful data to a file, classically known as the "Resource Fork". In the pre-OS X days applications were often bundled by placing images, audio, etc. in the application file's resource fork. I remember in the 90's finding an application that had a huge (for its day) multi-megabyte resource fork, but only a few kilobytes of program data in the data fork.
That is what the "_macosx" folder contains, only these days the resource fork is only used to store cached data such as thumbnails, window positions, etc. But it didn't always use to be that way, and for compatibility's sake we're stuck :(
It doesn't matter if modern programs can read _macosx, the problem is modern applications creating them. Since they store useless metadata these days, there is no reason not to stop default creation of them.
> I remember in the 90's finding an application that had a huge (for its day) multi-megabyte resource fork, but only a few kilobytes of program data in the data fork.
Heck, it wasn't uncommon for applications to have no data fork at all prior to the PowerPC transition. (PPC applications stored their object code in the data fork.)
That is what the "_macosx" folder contains, only these days the resource fork is only used to store cached data such as thumbnails, window positions, etc. But it didn't always use to be that way, and for compatibility's sake we're stuck :(